mmg: Add missing copyright header.

inria/mmg.scm

+;;; This module extends GNU Guix and is licensed under the same terms, those
+;;; of the GNU GPL version 3 or (at your option) any later version.
+;;;
+;;; Copyright © 2019 Inria
+
 (define-module (inria mmg)
   #:use-module (guix packages)
   #:use-module (guix download)