.mailmap: add entry for Matt DeVillier

... for git-shortlog purposes.

Cc: Matt DeVillier <matt.devillier@gmail.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Matt DeVillier <matt.devillier@gmail.com>