sysutils/lsof: fix build on -CURRENT.

sys/_lock.h is no longer implicitly included in such a way that the #ifdef
_KERNEL #error in lockmgr.h is bypassed after r347984. Fix lsof to explicitly
include _lock.h with _KERNEL defined on 13.

Submitted by: cem
Reported by: pkg-fallout
Differential Revision: https://reviews.freebsd.org/D20341