server: free strduped port numbers

Although the leak is negligible, the clean heap on exit will ease
valgrind testing.

Change-Id: I3a7a9c8e8dc7557aa51d0b9caa244537e5e7007d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4410
Tested-by: jenkins

src/server/gdb_server.h

@@ -36,6 +36,7 @@ struct reg;
 
 int gdb_target_add_all(struct target *target);
 int gdb_register_commands(struct command_context *command_context);
+void gdb_service_free(void);
 
 int gdb_put_packet(struct connection *connection, char *buffer, int len);