fix a typo in cube example

Change-Id: I1ad01aa519007d203669ccc63ef7d769b7e740b5
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>

examples/opengl/cube/main.cpp

@@ -50,7 +50,7 @@ int main(int argc, char *argv[])
     QApplication app(argc, argv);
     app.setApplicationName("cube");
     app.setApplicationVersion("0.1");
-#ifdef QT_NO_OPENGL
+#ifndef QT_NO_OPENGL
     MainWidget widget;
     widget.show();
 #else