security/tor: Unbreaking on systems without timingsafe_memcmp(3)

timingsafe_memcmp(3) has only been introduced in 12, therefore limiting its use to 12.

Reported by: Martin Gehlen <zitlo@zitlo.de> (bug reporter)
Approved by: adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D13502