gsettings: Install gettext ITS rules

Recent gettext has a feature to allow consumer projects to supply their
own string extraction rules for XML files, in ITS format.

Gettext still ships the rule for *.gschema.xml, but it would be better
maintained in the upstream project.

See the gettext documentation for details:
http://www.gnu.org/software/gettext/manual/html_node/Preparing-ITS-Rules.html

https://bugzilla.gnome.org/show_bug.cgi?id=760199