Qt: Hide debug menu by default

src/duckstation-qt/mainwindow.h

@@ -29,6 +29,10 @@ public:
   /// Performs update check if enabled in settings.
   void startupUpdateCheck();
 
+public Q_SLOTS:
+  /// Updates debug menu visibility (hides if disabled).
+  void updateDebugMenuVisibility();
+
 private Q_SLOTS:
   void reportError(const QString& message);
   void reportMessage(const QString& message);