bnxt_en: fix module Makefile for LinuxKPI

Rather than manually including the include directory for LinuxKPI
use the provided macro. Before there was no -I for the dummy
directory and as files synced from Linux under a permissive license
may include them the build would fail. overflow.h will include
linux/const.h which only exists as a dummy header at this point
on FreeBSD.

Sponsored by: The FreeBSD Foundation
Fixes: 35b53f8c989f6
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D56424

(cherry picked from commit f5cd992fa902ebb27a3a1042ee8e0d4d351bb361)