Add pdrfork to OpenBSM

It's necessary for auditd to be able to log its events. Note that the
recently added pdwait() syscall is only auditable because an audit event
of that name was preemptively added back in 2012.

PR: 292739
MFC after: 1 week
Fixes: 5c2ee618d5e "sys: add pdrfork(2)"
Sponsored by: ConnectWise
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D54897