Revert "package/weston: libxkbcommon is optional"

This reverts commit cf1c2eb1.

xkbcommon is still needed for the clients. There's no point in disabling
the clients, or weston is unusable (as packaged in Buildroot.)

Fixes:

  http://autobuild.buildroot.org/results/4e9/4e996c65f5b33d4518b0596d9c7076083d491a52/



Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>