sdp_msg.c: fix missing rate in sdp_audio_codecs_add()

When a new entry is is added by sdp_audio_codecs_add(), also add the
proper rate.

Change-Id: I17ad8862f662975e2df72a332eb83df7fb048cad
diff --git a/src/libmsc/sdp_msg.c b/src/libmsc/sdp_msg.c
index 49fc5d2..dba1541 100644
--- a/src/libmsc/sdp_msg.c
+++ b/src/libmsc/sdp_msg.c
@@ -154,6 +154,7 @@
 	OSMO_STRLCPY_ARRAY(codec->subtype_name, subtype_name);
 	if (fmtp)
 		OSMO_STRLCPY_ARRAY(codec->fmtp, fmtp);
+	codec->rate = rate;
 	return codec;
 }