fix: bring back official gettext repo

The mirror is no longer in sync.

Author: nijel

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "modules/gettext"]
 	path = modules/gettext
-	url = https://github.com/autotools-mirror/gettext.git
+	url = https://https.git.savannah.gnu.org/git/gettext.git
 [submodule "modules/l10n-guide"]
 	path = modules/l10n-guide
 	url = https://github.com/translate/l10n-guide.git