its: Support DocBook XML, part 1.

its: Support DocBook XML, part 1.

Reported by Guillem Jover <guillem@hadrons.org>
at <https://savannah.gnu.org/bugs/?52415>.

* gettext-tools/its/docbook4.its: New file, copied from
https://github.com/itstool/itstool file its/docbook.its.
* gettext-tools/its/docbook5.its: New file, copied from
https://github.com/itstool/itstool file its/docbook5.its.
* gettext-tools/its/docbook.loc: New file.
* gettext-tools/its/Makefile.am (ITS_FILES): Add docbook4.its, docbook5.its.
(LOC_FILES): Add docbook.loc.