security/yapet: fix build with GCC-based architectures

Don't link with $ssldir/lib if $ssldir is /usr.

This creates a build failure on GCC architectures. -L/usr/lib is added to LDFLAGS and GCC from ports tries to link to base libstdc++.

PR: 237703
Approved by: mat (maintainer), rafi@guengel.ch (maintainer timeout)
Differential Revision: https://reviews.freebsd.org/D20284