LinuxKPI: add the "dummy" includes directory to builds

While we could add the dummy includes directory manually to only the
drivers needing it, it seems a lot easier to simply add it to all
without any expected harm.

This is needed for more drivers (and to remove some #ifdef in current
ones) with empty header files being present not yielding errors.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: hselasky, imp
Differential Revision: https://reviews.freebsd.org/D36684