fix build with gcc 10

Define curr_state as extern in nanocom.h to avoid the following build
failure with gcc 10 (which defaults to -fno-common):

/home/buildroot/autobuild/instance-3/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: /tmp/ccLQeiek.o:(.bss+0x7c): multiple definition of `curr_state'; /tmp/ccse155z.o:(.bss+0x0): first defined here

Fixes:
 - http://autobuild.buildroot.org/results/4af4710cb9bbb1bc770b9824339dd7dbf8a80b05

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

diff -Naurp nanocom-1.0.orig/nanocom.c nanocom-1.0/nanocom.c
--- nanocom-1.0.orig/nanocom.c	2020-09-04 14:50:15.973320870 +0200
+++ nanocom-1.0/nanocom.c	2020-09-04 14:53:49.429325087 +0200
@@ -20,6 +20,8 @@ Based upon microcom by Anca and Lucian J
 
 #include "nanocom.h"
 
+state curr_state;
+
 int crnl_mapping; //0 - no mapping, 1 mapping
 
 char device[MAX_DEVICE_NAME]; /* serial device name */
diff -Naurp nanocom-1.0.orig/nanocom.h nanocom-1.0/nanocom.h
--- nanocom-1.0.orig/nanocom.h	2020-09-04 14:50:15.973320870 +0200
+++ nanocom-1.0/nanocom.h	2020-09-04 14:51:12.269321982 +0200
@@ -73,7 +73,7 @@ typedef struct {
     char echo_type; /*r for remote, l for local and n for none*/
 } state;
 
-state curr_state;
+extern state curr_state;
    
 #endif /* NANOCOM_H */