Fix missing return in QtOpenGl/paintedwindow example.

Change-Id: I17da0e93bb7c1b0cdbb5b76035ec913cbc616608
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

examples/opengl/paintedwindow/main.cpp

@@ -50,6 +50,6 @@ int main(int argc, char **argv)
     PaintedWindow window;
     window.show();
 
-    app.exec();
+    return app.exec();
 }