Add a new osmo_rtp_set_source_desc() function to set the RTCP SDES items

This allows the users of libosmotrau to set the content of the SDES
items to something more useful than their compile-time defaults.

Change-Id: Ice794f9e0c6caeea1c67520c12efbfa375d1fb82
Related: OS#2701
diff --git a/TODO-RELEASE b/TODO-RELEASE
index d0852fc..b02e1eb 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -7,3 +7,4 @@
 # If any interfaces have been added since the last public release: c:r:a + 1.
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what		description / commit summary line
+libosmotrau	addition	New osmo_rtp_set_source_desc() wrapper around libortp