qt: fix webkit dependencies

fix:
http://autobuild.buildroot.net/results/ce8/ce89ad1ca6aec9f7bb5549301bf4c0dcba9e0e37/



qtwebkit includes qscript headers.
For packages like python-pyqt if webkit is available but not qscript, the
compilation fails with :
buildroot/output/build/python-pyqt-4.11.3/sip/QtWebKit/qwebframe.sip:79:0:
../../../host/usr/i686-buildroot-linux-gnu/sysroot/usr/include/QtWebKit/qwebframe.h:28:36:
	fatal error: QtScript/qscriptengine.h: No such file or directory

This problem is not seen with Qt compilation because some relative include
directly from sources are added.

[Peter: also adjust comment]
Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>