package/libxkbcommon: fix builds with/without X11

We need to explicitly disable X11 support when X11 is not available.
Conversely, when it is, we need to add the required dependencies.

Fixes a slew of autobuild failures:
    http://autobuild.buildroot.org/results/587/58794f894a4950b77e38d5fc877852d6cc507d8c/
    http://autobuild.buildroot.org/results/927/92741f6b9b7909b226c200e453ac6cee8af5abd6/
    http://autobuild.buildroot.org/results/51c/51cb09f7f267319948c685f905e29afe99f59589/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Fatih Aşıcı <fatih.asici@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>