autotools: Add the Meson files to the dist tarballs

We'd like everybody to build Gom with Meson in the near-future.

However, for now we're still generating the release tarballs with the
Autotools and their `make distcheck` target.

This change makes sure tarballs contain the essentials to be built with
Meson.

https://bugzilla.gnome.org/show_bug.cgi?id=781944