blocklistd-helper: Silence another bogus pf warning

It has been reported as PR 290478. In the meantime, just sweep under
the carpet.

It is worth noting that neither commit:
2347ca21d657 ("blocklist-helper: Silence a bogus pf warning")
nor this one will be upstreamed, as this is a FreeBSD-specific issue.

PR: 290478
MFC after: 2 days