LinuxKPI: add linux/bsearch.h for sort(9)

Add linux/bsearch.h which only includes libkern.h as the sort(9)
functions seem to be compatible.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30417

(cherry picked from commit f4a145b136ad88f7a9f38e3f7732bc5a8b5cb517)