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/server/gdb_server.h

@@ -20,6 +20,7 @@
 struct image;
 struct reg;
 #include <target/target.h>
+#include <server/server.h>
 
 #define GDB_BUFFER_SIZE 16384
 
@@ -29,6 +30,8 @@ void gdb_service_free(void);
 
 int gdb_put_packet(struct connection *connection, char *buffer, int len);
 
+int gdb_get_actual_connections(void);
+
 static inline struct target *get_target_from_connection(struct connection *connection)
 {
 	struct gdb_service *gdb_service = connection->service->priv;