Fixed unused parameter warning in hellowindow example.

Change-Id: I48ea19b5548a179d8116685fa481c4d1b3804bba
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>

examples/opengl/hellowindow/hellowindow.cpp

@@ -80,6 +80,8 @@ HelloWindow::HelloWindow(Renderer *renderer)
 
 void HelloWindow::exposeEvent(QExposeEvent *event)
 {
+    Q_UNUSED(event);
+
     render();
 
     QTimer *timer = new QTimer(this);