osmux: account extracted traffic from the osmux batch

Use struct mgcp_rtp_end statistics to account the RTP messages
that has been extracted from the osmux batch and transmitted.
diff --git a/openbsc/src/libmgcp/mgcp_osmux.c b/openbsc/src/libmgcp/mgcp_osmux.c
index c285de4..0fa7b45 100644
--- a/openbsc/src/libmgcp/mgcp_osmux.c
+++ b/openbsc/src/libmgcp/mgcp_osmux.c
@@ -216,6 +216,9 @@
 		.sin_port = endp->net_end.rtp_port,
 	};
 
+	endp->bts_end.octets += msg->len;
+	endp->bts_end.packets++;
+
 	mgcp_send(endp, MGCP_DEST_NET, 1, &addr, (char *)msg->data, msg->len);
 	msgb_free(msg);
 }
@@ -228,6 +231,9 @@
 		.sin_port = endp->bts_end.rtp_port,
 	};
 
+	endp->net_end.octets += msg->len;
+	endp->net_end.packets++;
+
 	mgcp_send(endp, MGCP_DEST_BTS, 1, &addr, (char *)msg->data, msg->len);
 	msgb_free(msg);
 }