security/openssh-portable: Set LICENSE

In the past, the ports framework did not support handling situations
where a port contained a multitude 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.

I am not bumping PORTREVISION as it is not a vital change from the
perspective of package users.

Approved by: bdrewery (maintainer)
Differential Revision: https://reviews.freebsd.org/D27133