mgcp: Store one more codec/payload type if it is present

In case of some RTP proxy from time to time we are offered both
G729 and G711 but only one of them will work. I intend to adjust
the codec at runtime in case we receive the wrong codec.
diff --git a/openbsc/include/openbsc/mgcp_internal.h b/openbsc/include/openbsc/mgcp_internal.h
index 2458909..34c3d97 100644
--- a/openbsc/include/openbsc/mgcp_internal.h
+++ b/openbsc/include/openbsc/mgcp_internal.h
@@ -90,6 +90,7 @@
 
 	/* audio codec information */
 	struct mgcp_rtp_codec codec;
+	struct mgcp_rtp_codec alt_codec; /* TODO/XXX: make it generic */
 
 	/* per endpoint data */
 	int  frames_per_packet;