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).
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The migration is almost done, at least the rest should happen in the background. There are still a few technical difference between the old cluster and the new ones, and they are summarized in this issue. Please pay attention to the TL:DR at the end of the comment.
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).