Fix compiler warning in example

Fix warning

main.cpp:160:
warning: no return statement in function returning non-void
[-Wreturn-type]
 }
  ^

Change-Id: I4b74e8f56136077ef9a71b803d2756394fcd7658
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

examples/gui/analogclock/main.cpp

@@ -156,5 +156,5 @@ int main(int argc, char **argv)
     AnalogClockWindow clock;
     clock.show();
 
-    app.exec();
+    return app.exec();
 }