linuxkpi: Add linux/stackdepot.h

With a typedef needed by drm-kmod.

Reviewed by: bz
Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36105

(cherry picked from commit b1c82bd4026122e2b3dc4d18005069873da6b246)