MNCC: introduce mncc_upq_enqueue() wrapper around msgb_enqueue()

This wrapper will allow us to add queue related code at one single
location in the code as opposed to three locations.
diff --git a/openbsc/src/rtp_proxy.c b/openbsc/src/rtp_proxy.c
index a1f440d..a2a9107 100644
--- a/openbsc/src/rtp_proxy.c
+++ b/openbsc/src/rtp_proxy.c
@@ -443,7 +443,7 @@
 		if (rc < 0)
 			goto out_free;
 		msgb_free(msg);
-		msgb_enqueue(&rs->receive.net->upqueue, new_msg);
+		mncc_upq_enqueue(rs->receive.net, new_msg);
 		break;
 
 	case RTP_NONE: /* if socket exists, but disabled by app */