Fix warning about virtual override

Change-Id: Ia9928c8ef5d1597e55f45c67c462c1e9756ee899
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp

@@ -404,7 +404,7 @@ void tst_QThreadPool::expiryTimeoutRace() // QTBUG-3786
 class ExceptionTask : public QRunnable
 {
 public:
-    void run()
+    void run() override
     {
         throw new int;
     }