capsicum.h: Include ktrace.h only in kernel

Fix cross build failure by including ktrace.h only when _KERNEL is
defined.

Fixes: 9bec84131215
Approved by: markj (mentor)
MFC after: 1 month
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 34791f4ac79ee41f6986178a3a89d13f37ac156c)