unix: don't include CoreServices globally on macOS
unix: don't include CoreServices globally on macOS

In unix/internal.h, CoreServices is included for AvailabilityMacros.h.
This commit just includes AvailabilityMacros.h directly instead.

PR-URL: https://github.com/libuv/libuv/pull/1092
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

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