mlx5 / mlxfw: make depend on compat_linuxkpi

mlx5 and mlxfw both depend on LinuxKPI also being compiled into the
kernel. With nooptions COMPAT_LINUXKPI (LinuxKPI only compiled as
module) mlx5 and mlxfw fail to link due to undefined symbols.

Add the missing compat_linuxkpi to the files file.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D49737