Build the cube opengl example both for desktop and ES

No reason to restrict it to desktop only, the code works on ES2 too.

Task-number: QTBUG-35730

Change-Id: I915a88cbf3c95a941d6bafd88ed57fef0b6a25b9
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>