Make xkbcommon mandatory

Make xkbcommon mandatory

We really need xkbcommon. There is so much stuff (including parsing of
keyboard shortcuts in conf.c) that depends on it. Therefore, we make it
mandatory now which allows us to use xkbcommon functions all over the
place.

Note that xkbcommon itself has no runtime dependencies so it is a small
self-contained library. The only reason I didn't do this ealier is that
xkbcommon has not seen a public release, yet. However, that should be done
in the near future.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>