r316063 installed pf's embedded libevent as a private lib, with headers
in /usr/include/private/event. Unfortunately we also have a copy of
libevent v2 included in ntp, which needed to be updated for compatibility
with OpenSSL 1.1.
As unadorned 'libevent' generally refers to libevent v2, be explicit that
this one is libevent v1.
Reviewed by: vangyzen (earlier)
Differential Revision: https://reviews.freebsd.org/D17275
