Address review comments

src/yuzu/multiplayer/lobby.h

@@ -9,7 +9,6 @@
 #include <QSortFilterProxyModel>
 #include <QStandardItemModel>
 #include "common/announce_multiplayer_room.h"
-#include "core/core.h"
 #include "network/announce_multiplayer_session.h"
 #include "network/network.h"
 #include "yuzu/multiplayer/validation.h"
@@ -21,6 +20,10 @@ class Lobby;
 class LobbyModel;
 class LobbyFilterProxyModel;
 
+namespace Core {
+class System;
+}
+
 /**
  * Listing of all public games pulled from services. The lobby should be simple enough for users to
  * find the game they want to play, and join it.