Compile fix for Mac OS X 10.7 with 10.6 sdk

Since it is an example it is cleaner to just include the header file
as it will be available on other platforms anyway.  Also ensured that
the Qt includes were first before the system ones.

Task-number: QTBUG-22641

Merge-request: 1462
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

examples/opengl/cube/mainwidget.cpp

@@ -46,10 +46,10 @@
 
 #include <QBasicTimer>
 #include <QMouseEvent>
+#include <QDebug>
 
 #include <math.h>
-
-#include <QDebug>
+#include <locale.h>
 
 MainWidget::MainWidget(QWidget *parent) :
     QGLWidget(parent),