gst/rtpmanager/gstrtpsession.c: Use method to get the internal SSRC.

gst/rtpmanager/gstrtpsession.c: Use method to get the internal SSRC.

Original commit message from CVS:
* gst/rtpmanager/gstrtpsession.c:
(gst_rtp_session_getcaps_send_rtp):
Use method to get the internal SSRC.
* gst/rtpmanager/rtpsession.c: (rtp_session_class_init),
(rtp_session_set_property), (rtp_session_get_property):
Add property to congiure the internal SSRC of the session.
Fixes #565910.