linuxkpi: Include highmem.h in pagemap.h

Linux does the same.

Reviewed by: bz, emaste, hselasky
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36023

(cherry picked from commit b829f450a7a3915d21031b69e83e535c1bd3248d)