Qt/CheatWarningWidget: properly supply a parent for the widget

Because it wasn't parented properly, it would show briefly the widget in its own window when creating an ARCodeWidget or a GeckoCodeWidget which would occur when accessing the game properties page or when the state changes to pause/running.

Source/Core/DolphinQt2/Config/CheatWarningWidget.h

@@ -15,7 +15,7 @@ class CheatWarningWidget : public QWidget
 {
   Q_OBJECT
 public:
-  explicit CheatWarningWidget(const std::string& game_id, bool restart_required);
+  explicit CheatWarningWidget(const std::string& game_id, bool restart_required, QWidget* parent);
 
 signals:
   void OpenCheatEnableSettings();