fix build with gcc 10

gcc 10 is now defaulting to -fno-common resulting in the following build
failures:

/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/bin/arm-buildroot-linux-gnueabihf-gcc  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os -g2 -D_FORTIFY_SOURCE=1 -DENABLE_IPV6 -DD_DNS_THREADED -D_REENTRANT -D_THREAD_SAFE -Wall -W -Wno-unused-parameter -Waggregate-return   -o dpid dpi.o dpi_socket_dir.o dpid.o dpid_common.o main.o misc_new.o ../dpip/libDpip.a ../dlib/libDlib.a 
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: dpi_socket_dir.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: multiple definition of `dpi_errno'; dpi.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: dpid.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: multiple definition of `dpi_errno'; dpi.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: dpid_common.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: multiple definition of `dpi_errno'; dpi.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: main.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:64: multiple definition of `sock_set'; dpid.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:64: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: main.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:61: multiple definition of `services_list'; dpid.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:61: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: main.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:58: multiple definition of `dpi_attr_list'; dpid.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:58: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: main.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:55: multiple definition of `numsocks'; dpid.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:55: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: main.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:52: multiple definition of `numdpis'; dpid.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:52: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: main.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:31: multiple definition of `srs_fd'; dpid.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:31: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: main.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:28: multiple definition of `srs_name'; dpid.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid.h:28: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: main.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: multiple definition of `dpi_errno'; dpi.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: misc_new.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: multiple definition of `dpi_errno'; dpi.o:/nvme/rc-buildroot-test/scripts/instance-0/output-1/build/dillo-3.0.5/dpid/dpid_common.h:44: first defined here

So add missing extern for srs_fd, numdpis, numsocks, dpi_attr_list,
services_list and sock_set

Also remove srs_name which is never used and give a name to dpi_errno
enum

Fixes:
 - http://autobuild.buildroot.org/results/9c777af97fe50143c6a68f0170fc86c87d8ead3f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Upstream status: sent to jcid@dillo.org]

diff -r f60d55c02567 dpid/dpid.c
--- a/dpid/dpid.c	Fri Oct 05 22:05:08 2018 +0200
+++ b/dpid/dpid.c	Thu Aug 27 18:16:50 2020 +0200
@@ -38,6 +38,12 @@
 
 #define QUEUE 5
 
+int srs_fd;
+int numdpis;
+int numsocks;
+struct dp *dpi_attr_list;
+Dlist *services_list;
+fd_set sock_set;
 volatile sig_atomic_t caught_sigchld = 0;
 char *SharedKey = NULL;
 
diff -r f60d55c02567 dpid/dpid.h
--- a/dpid/dpid.h	Fri Oct 05 22:05:08 2018 +0200
+++ b/dpid/dpid.h	Thu Aug 27 18:16:50 2020 +0200
@@ -25,10 +25,9 @@
 
 /*! \TODO: Should read this from dillorc */
 #define SRS_NAME "dpid.srs"
-char *srs_name;
 
 /*! dpid's service request socket file descriptor */
-int srs_fd;
+extern int srs_fd;
 
 /*! plugin state information
  */
@@ -49,19 +48,19 @@
 };
 
 /*! Number of available plugins */
-int numdpis;
+extern int numdpis;
 
 /*! Number of sockets being watched */
-int numsocks;
+extern int numsocks;
 
 /*! State information for each plugin. */
-struct dp *dpi_attr_list;
+extern struct dp *dpi_attr_list;
 
 /*! service served for each plugin  */
-Dlist *services_list;
+extern Dlist *services_list;
 
 /*! Set of sockets watched for connections */
-fd_set sock_set;
+extern fd_set sock_set;
 
 /*! Set to 1 by the SIGCHLD handler dpi_sigchld */
 extern volatile sig_atomic_t caught_sigchld;
diff -r f60d55c02567 dpid/dpid_common.h
--- a/dpid/dpid_common.h	Fri Oct 05 22:05:08 2018 +0200
+++ b/dpid/dpid_common.h	Thu Aug 27 18:16:50 2020 +0200
@@ -37,10 +37,10 @@
 
 
 /*! Error codes for dpid */
-enum {
+enum dpi_errno {
    no_errors,
    dpid_srs_addrinuse /* dpid service request socket address already in use */
-} dpi_errno;
+};
 
 /*! Intended for identifying dillo plugins
  * and related files
diff -r f60d55c02567 dpid/main.c
--- a/dpid/main.c	Fri Oct 05 22:05:08 2018 +0200
+++ b/dpid/main.c	Thu Aug 27 18:16:50 2020 +0200
@@ -231,7 +231,7 @@
    /* this sleep used to unmask a race condition */
    // sleep(2);
 
-   dpi_errno = no_errors;
+   enum dpi_errno error_code = no_errors;
 
    /* Get list of available dpis */
    numdpis = register_all(&dpi_attr_list);
@@ -254,7 +254,7 @@
 
    /* Initialise sockets */
    if ((numsocks = init_ids_srs_socket()) == -1) {
-      switch (dpi_errno) {
+      switch (error_code) {
       case dpid_srs_addrinuse:
          MSG_ERR("dpid refuses to start, possibly because:\n");
          MSG_ERR("\t1) An instance of dpid is already running.\n");