webkit: disable on powerpc

webkit does not build correctly on powerpc. There is an upstream bug report
with a partial fix, but with remaining problems.
https://bugs.webkit.org/show_bug.cgi?id=113638

Until this problem is fixed, we disable webkit on powerpc.

Fixes
http://autobuild.buildroot.org/results/3ce/3ce992a663ddf1030a987bb778f8275a8e5fdef0/



Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>