linuxkpi: Include <linux/shinker.h> in <linux/mm.h>

This is done like this in Linux.

Reviewed by: emaste manu
Approved by: emaste manu
Differential Revision: https://reviews.freebsd.org/D36956

(cherry picked from commit 415ca4d27d76e0787d65108fd96c980ea68c6eff)