Fix Windows build with --enable-werror on Ubuntu Focal

src/qt/winshutdownmonitor.h

@@ -17,7 +17,7 @@ class WinShutdownMonitor : public QAbstractNativeEventFilter
 {
 public:
     /** Implements QAbstractNativeEventFilter interface for processing Windows messages */
-    bool nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult);
+    bool nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult) override;
 
     /** Register the reason for blocking shutdown on Windows to allow clean client exit */
     static void registerShutdownBlockReason(const QString& strReason, const HWND& mainWinId);