net/openssh: (hopefully) fix build failures on 10.6-. Round three.
Security/SecTask.h is part of libsecurity_codesigning (and might have
seen quite some moves around the tree during its lifetime, including a
move back into the Security framework in a sectask subdirectory), but
not available on 10.6-. We don't actually need it, so drop it on 10.6-.
No revbump since this only fixes build failures.
Fixes: https://trac.macports.org/ticket/59397
