The KERN_OPTS:MDEV_ACPI checks are fine for as long as we are building
modules along the kernel. If one wants to just build the module
standalone out of the module directory this would fail.
Add the missing include for kmod.opts.mk (as was done for tcp
in 1319a76179682).
Sponsored by: The FreeBSD Foundation
Reported by: Tassilo Philipp (tphilipp potion-studios.com)
Fixes: f5a77dc8f8df ("improve module Makefile dependency on ACPI")