Use unmodified QtSingleApplication on Windows.

Get running application instance PID without QtSingleApplication
modification.

src/app/qtsingleapplication/qtlocalpeer.h

@@ -57,9 +57,6 @@ public:
     bool sendMessage(const QString &message, int timeout);
     QString applicationId() const
         { return id; }
-#ifdef Q_OS_WIN
-    qint64 getRunningPid();
-#endif
 
 Q_SIGNALS:
     void messageReceived(const QString &message);