Fix no-MMU build

The detach_from_terminal() is unused for no-MMU, but it depends on symbols
that are not defined for no-MMU. Don't define detach_from_terminal() when
HAVE_WORKING_FORK is not defined.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
---
Upstream status: http://bugs.ntp.org/show_bug.cgi?id=3538

diff -Nuar ntp-4.2.8p12.orig/ntpd/ntpd.c ntp-4.2.8p12/ntpd/ntpd.c
--- ntp-4.2.8p12.orig/ntpd/ntpd.c	2018-08-14 14:51:30.000000000 +0300
+++ ntp-4.2.8p12/ntpd/ntpd.c	2018-10-13 21:25:25.858261249 +0300
@@ -534,6 +534,7 @@
  * Detach from terminal (much like daemon())
  * Nothe that this function calls exit()
  */
+# ifdef HAVE_WORKING_FORK
 static void
 detach_from_terminal(
 	int pipe_fds[2],
@@ -617,6 +618,7 @@
 
 	return;
 }
+# endif /* HAVE_WORKING_FORK */
 
 #ifdef HAVE_DROPROOT
 /*