buildenv/osx: update libsndfile's .pc file to use Requires.private.
buildenv/osx: update libsndfile's .pc file to use Requires.private.

...Instead of Libs.private.

This worked when Ogg, Vorbis and FLAC would spell out their dependencies
explicitly in Libs (or its .private equivalent) as well, but that's not
the case any more. They now use Requires.private.

ccpgames/mumble-releng,ccpgames/mumble-releng