Get the Windows GL context to compile.

Not sure why this was not the case all along, but on my MSVC2008
installation the include must be prefixed by the 'GL' directory.

Change-Id: Icb29b072446bab386b5c62ab0df453ded41b8ede
Reviewed-on: http://codereview.qt-project.org/4914
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

src/plugins/platforms/windows/qwindowsglcontext.cpp

@@ -47,11 +47,7 @@
 #include <QtCore/QSysInfo>
 
 #include <WinGDI.h>
-#if defined(Q_CC_MINGW)
-#    include <GL/Gl.h>
-#else
-#    include <Gl.h>
-#endif
+#include <GL/Gl.h>
 
 // #define DEBUG_GL