gettext: Add macOS warning patch

gettext 0.20 fixed a bug with handling locale on macOS, but this caused
it to report an annoying warning on systems where “language”
differs from “region”. See Homebrew issue for details:
<https://github.com/Homebrew/homebrew-core/issues/41139>.

Add upstream patch that has not been released yet.
Details:
<https://www.mail-archive.com/bug-gnulib@gnu.org/msg36768.html>.