security/openssh-portable: Set LICENSE

In the past, the ports framework did not support handling situations
where a port contains multitiude of licenses. In case of OpenSSH
the list is/was: BSD2, BSD3, MIT, public domain, BSD-Style, BEER-WARE,
"any purpose with notice intact", and ISC-Style.

Instead of having to keep track  of all the involved licenses which all
are very similar, let's use LICENSE_PERMS.