New dep required for GNOME Software git master

[Richard Hughes <hughsient gmail com>]

For the gnome-software 3.31.x unstable tarballs I'm soon going to be
adding a hard dependency on a new project
https://github.com/hughsie/libxmlb -- it's intended to be a
replacement for appstream-glib. I've written a bit more about it here:

https://blogs.gnome.org/hughsie/2018/09/20/speeding-up-appstream-mmaping-xml-using-libxmlb/
https://blogs.gnome.org/hughsie/2018/10/04/announcing-the-first-release-of-libxmlb/

Tarballs are here: https://people.freedesktop.org/~hughsient/releases/
-- it's a tiny meson library that uses gobject introspection, so it
should be trivial to package.

I figured it was a good idea to post here so that distros would have
time to audit/package/complain before the hard dep is added. fwupd
will also require the dep in a few weeks time too. Fedora 29/30
already has packages if that helps.

Richard.