rtpdec: Use our own SSRC in the SDES field when sending RRs

rtpdec: Use our own SSRC in the SDES field when sending RRs

The s->ssrc field is the sender's SSRC, we use ssrc + 1 to get
a collision free "unique" SSRC for ourselves in the RR part.
The SDES block in the RTCP packet should describe ourselves,
not the sender.

This was fixed for the RR part in 952139a3226b, but wasn't
fixed for the SDES part until now.

This could cause some Axis cameras to send RTCP BYE packets
to us due to the SSRC collision.

Signed-off-by: Martin Storsjö <martin@martin.st>