server: gdb: export gdb_actual_connections through a function

The internal variable 'gdb_actual_connections' is used by log and
by semihosting to determine if there are active GDB connections.

Keep the variable local in server's code and only export its value
through a dedicated function.

This solves the issue detected by 'parse' of the variable defined
as global but not declared in any include file.

Change-Id: I6e14f4cb1097787404094636f8a2a291340222dd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7673
Tested-by: jenkins

src/helper/log.c

@@ -19,6 +19,7 @@
 #include "command.h"
 #include "replacements.h"
 #include "time_support.h"
+#include <server/gdb_server.h>
 #include <server/server.h>
 
 #include <stdarg.h>
@@ -399,9 +400,7 @@ char *alloc_printf(const char *format, ...)
 
 static void gdb_timeout_warning(int64_t delta_time)
 {
-	extern int gdb_actual_connections;
-
-	if (gdb_actual_connections)
+	if (gdb_get_actual_connections())
 		LOG_WARNING("keep_alive() was not invoked in the "
 			"%d ms timelimit. GDB alive packet not "
 			"sent! (%" PRId64 " ms). Workaround: increase "