linuxkpi: Add linux/vgaarb.h
Needed by drm-kmod.
Reviewed by: bz
Obtained from: Linux
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36104
(cherry picked from commit 4b4ab8c3107f8a95db76a7dfe50485715c8b3703)