xkb: rename xkb.h to xkb-procs.h

This header merely defines the various protocol request handlers, so
let's rename it to something less generic and remove its include from
all the files that don't actually need it (which is almost all of them).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>