Fix the build of fork-running-state.c on NetBSD

Include <signal.h> for kill(2).

gdb/testsuite/ChangeLog:

        * gdb.base/fork-running-state.c: Include "signal.h".

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-04-17  Kamil Rytarowski  <n54@gmx.com>
+
+	* gdb.base/fork-running-state.c: Include "signal.h".
+
 2020-04-17  Tom Tromey  <tromey@adacore.com>
 
 	* gdb.cp/maint.exp (test_help): Simplify multiple_help_body.

gdb/testsuite/gdb.base/fork-running-state.c

@@ -20,6 +20,7 @@
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <errno.h>
+#include <signal.h>
 
 int save_parent;