ctl: Make ctl_private.h more self-contained

Include <sys/sysctl.h> for sysctl context types.

Reviewed by: ken, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D42209

(cherry picked from commit fc8cf0a8deb8908fb17acc56e96f3f700c05136d)