LinuxKPI: always include linux/kconfig.h

Always include linux/kconfig.h which seems to match Linux behaviour
and avoid errors compiling code expected from that file but never
included.

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste, imp
Differential Revision: https://reviews.freebsd.org/D46801

(cherry picked from commit 41283b454b7abd3de251ec8234d19b67dbca3916)