pf: fix LINT build

We failed to list the new pf_syncookies.c file in sys/conf/files. This
worked for the usual configurations, where pf is a module, but not for
LINT builds.

Reported by: lwhsu
MFC after: 1 week
Sponsored by: Modirum MDPay

(cherry picked from commit b972a7fa9e1e01367435a5699b71cc7b5e494fee)