mupdf: Unbreak build on Darwin
mupdf: Unbreak build on Darwin

Recent changes (ea5dd0974d1e74d7338a8763b0f313bb09fa5ed9) dropped a
patch Darwin needed. I've added a new unconditional patch, because it
doesn't affect Linux and this way it's more obvious when the patch needs
updating.

I decided to add the `xcbuild` dependency because the upstream
build uses `xcrun` and I don't know of a good reason not to.

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