qt5webkit: Fix build without OpenGL

Fixes:
http://autobuild.buildroot.net/results/691/6918c623b65a3cee45f2c29c29b36f9bf475cc76/
http://autobuild.buildroot.net/results/914/914056cbd9787aeae4c8b42e8a37453b1a271a02/
http://autobuild.buildroot.net/results/84d/84d3285cc8e2669c4ff4c1be3a1e8ea9a6933779/

Backport patch from upstream git:
https://qt.gitorious.org/qt/qtwebkit/commit/4ecb913768ff0806c6efdff4567ef5907f597e4a



Signed-off-by: Johan Derycke <johan.derycke@barco.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>