net-mgmt/nrpe3: Fix build with LibreSSL
The patch enables the configure of the port to detect LibreSSL as a valid OpenSSL

Reported by: Philip Jocks <pj@netzkommune.de>
Reviewed by: brnrd, zeising (maintainer)
Approved by: zeising (maintainer)
Sponsored by: Netzkommune GmbH
Differential Revision: https://reviews.freebsd.org/D13421