Mk/bsd.sanity.mk: Prevent false positive with SSL_DEFAULT!=base after r499267

With SSL_DEFAULT!=base USES=ssl sets OPENSSL_LDFLAGS and any port with an
OPENSSL option like net/haproxy gets a false positive error like:

/!\ haproxy-1.9.8: Makefile errors /!\

The following options helpers are incorrectly set after bsd.port.options.mk
and are ineffective: OPENSSL_LDFLAGS

• Error code 1

Whitelist OPENSSL_LDFLAGS to prevent this.

Reported by: mfechner (via ports-committers)
Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D20275