Codechange: move all NetworkGameInfo related functions to a single file

It currently was a bit scattered over the place. Part of
NetworkGameInfo is also the GRF Identifiers that goes with it.

src/network/network_admin.cpp

@@ -10,6 +10,7 @@
 #include "../stdafx.h"
 #include "../strings_func.h"
 #include "../date_func.h"
+#include "core/game_info.h"
 #include "network_admin.h"
 #include "network_base.h"
 #include "network_server.h"