LinuxKPI: Add xen/xen.h header

It contains proxy-implementation of xen_initial_domain() and
xen_pv_domain() required by latest drm-kmod.

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42797

(cherry picked from commit c612e3c262db6c8256159c4ed39018788017c7e2)