Make qtwebkit disable itself if the gui module isn't present.

Building qtwebkit will fail if the qtgui modules is not present and
hence the qt build will fail.

Task-number: QTBUG-33108
Change-Id: Iab56781bf194e100daf3e509a5ed7e274ee201e4
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>