package/weston: libxkbcommon is optional

It is possible to build weston without libxkbcommon, for example
if using an input method that is not an hardware keyboard (e.g. a
virtual keyboard, or none at all.)

Make it optional.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>