res_pjsip_sdp_rtp: Allow only single ssrc attribute.

When processing SSRC attributes we were iterating through
all of them, even though we only need to know the remote
SSRC once. This was problematic because some browsers group
SSRCs together on a stream, and due to our negotiation only
end up using the first one. Since we set the second one as
the remote SSRC we would drop the received media from them
instead of allowing it through.

In the future this may be extended to allow SSRC groups
and to use information from the attributes.

Change-Id: I4dc87087dbe56a83aa65f0f897bbd4ca75ec1270