xkb: Print the xkbcomp path being executed when we fail to compile.

I don't know how many times I've had a broken server due to a bad
directory to xkbcomp, and only finding the whole path has shown me
where I went wrong.

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 30f4d440)