Qt: Fix window close in nogui mode

src/duckstation-qt/displaywidget.h

@@ -5,6 +5,8 @@
 #include <QtWidgets/QWidget>
 #include <optional>
 
+class QCloseEvent;
+
 class DisplayWidget final : public QWidget
 {
   Q_OBJECT
@@ -24,6 +26,8 @@ public:
   void updateRelativeMode(bool enabled);
   void updateCursor(bool hidden);
 
+  void handleCloseEvent(QCloseEvent* event);
+
 Q_SIGNALS:
   void windowResizedEvent(int width, int height, float scale);
   void windowRestoredEvent();