osmo_ortp.c: Fix warning: implicit use of function

Explicitly state that we are using an internal API from ortp, and
define the function we are using to avoid printing a warning.

Change-Id: I9cadcb31ce7ade3632d83753be97fdc9ea518b5b
diff --git a/src/trau/osmo_ortp.c b/src/trau/osmo_ortp.c
index 4e9df56..c4e9d53 100644
--- a/src/trau/osmo_ortp.c
+++ b/src/trau/osmo_ortp.c
@@ -199,6 +199,10 @@
 	return 0;
 }
 
+/* Internal API coming from rtpsession_priv.h, used in osmo_rtcp_fd_cb */
+#pragma message ("Using internal ortp API: rtp_session_rtcp_rec")
+int rtp_session_rtcp_recv(RtpSession * session);
+
 static int osmo_rtcp_fd_cb(struct osmo_fd *fd, unsigned int what)
 {
 	struct osmo_rtp_socket *rs = fd->data;
@@ -360,7 +364,7 @@
 	/* initialize according to the RFC */
 	rtp_session_set_seq_number(rs->sess, random());
 	rs->tx_timestamp = random();
-	
+
 
 	return rs;
 }