pkg-config: add vendored pc files to default path

Homebrew provides some missing pc files for recent OS X versions,
and they should be available to pkg-config by default.

Note that under both stdenv and superenv, we override the built-in path,
so changes to the formula do not affect the stability of builds.

cf. #21171.
Closes #24321.
pkg-config: update bottle.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>