mgcp: Calculate the wrap around as of Appendix A in RFC 3550

This is missing the probation and the dealing with a remote
restart. For the remote restart we will simply write a log
statement as this is unlikely to happen during a call or if
it does happen the call will be taken down by the BSC anyway.
diff --git a/openbsc/include/openbsc/mgcp_internal.h b/openbsc/include/openbsc/mgcp_internal.h
index e0086fa..f5e3cfc 100644
--- a/openbsc/include/openbsc/mgcp_internal.h
+++ b/openbsc/include/openbsc/mgcp_internal.h
@@ -50,6 +50,7 @@
 	uint16_t base_seq;
 	uint16_t max_seq;
 	int seq_offset;
+	int cycles;
 
 	uint32_t last_timestamp;
 	int32_t  timestamp_offset;