modules: Only build MAC modules if options MAC is set

They are not loadable otherwise.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D35369

(cherry picked from commit 648a47b85476ed0f8e3c61800b134bd3bb6001ee)