srtpenc: also insert ssrc(s) from rtp buffers
This fixes a regression from commit "srtp: Support libsrtp2" e9aa1172 where an internal set of ssrc(s) was added because the libsrtp v2 keep its internal streams as private. This patch ensures that all ssrc(s) are inserted to this set instead of only inserting those from the caps.
Edited by Julien Isorce