midori: depend on x11 backend for gtk3

Mirror commit ba25711e for midori as well, otherwise gtk3 with non-X11
backends can trigger build failures. Fixes:
http://autobuild.buildroot.net/results/80ffdf54fd675fbff5a645784cd041cdf862154c/
http://autobuild.buildroot.net/results/1c049845ade60de223296c77c8a70869c4db64f0/



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