net-mgmt, security: add missing SUBDIRs

net-mgmt/prometheus-bird-exporter and security/lfacme were missing
SUBDIR entries in their categories.

Reported by: Antoine Brodin <antoine@freebsd.org>
Reviewed by: arrowd, kevans
Approved by: arrowd (ports), kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D50976