Qt: Fix main window stuck open after update

src/duckstation-qt/mainwindow.h

@@ -87,6 +87,9 @@ public:
   /// Locks the system by pausing it, while a popup dialog is displayed.
   SystemLock pauseAndLockSystem();
 
+  /// Force quits the application.
+  void quit();
+
   /// Accessors for the status bar widgets, updated by the emulation thread.
   ALWAYS_INLINE QLabel* getStatusRendererWidget() const { return m_status_renderer_widget; }
   ALWAYS_INLINE QLabel* getStatusResolutionWidget() const { return m_status_resolution_widget; }