webkitgtk24: depend on x11 backend for gtk3

The other rendering backends aren't supported right now, so depend on
the X11 one to avoid build breakage. Fixes:
http://autobuild.buildroot.net/results/1b3/1b372dd04eaccd8a9d24390d509b0a49f692746c/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>