linuxkpi: Add <linux/pgtable.h>

For now, it's empty. It is included by the DRM drivers but nothing is
missing otherwise. Perhaps something we already defined should be in
pgtable.h instead of another header.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38153

(cherry picked from commit 7649b89cc12696d540b20ad2f5896827b102225e)