oggopus: Add a BSD license header.

This also removes the request for an additional grant from the IETF
 Trust on the final RFC.
I believe this is sufficient to exercise our ability to allow
 downstream modifications as proposed in RFC 5377 Section 4.4.
See the discussion at
 <https://www.ietf.org/mail-archive/web/codec/current/msg03169.html>
 for details.