Add CTRL interface to osmo-mgw

OsmoMGW has a lot of nice built-in statistics (rate_ctr,...) but it
seems the only way to look at them is via the VTY. While libosmocore
contains automatic exposure of all rate counters via CTRL, the CTRL
interface simply is not used by osmo-mgw so far.

Closes: OS#4441
Change-Id: I7ed6bdb9f4749c24ca11a5905a620546cfe42952
diff --git a/include/osmocom/mgcp/mgcp.h b/include/osmocom/mgcp/mgcp.h
index 71d6342..a479fbb 100644
--- a/include/osmocom/mgcp/mgcp.h
+++ b/include/osmocom/mgcp/mgcp.h
@@ -279,6 +279,9 @@
 
 	/* time after which inactive connections (CIs) get closed */
 	int conn_timeout;
+
+	/* osmocom CTRL interface */
+	struct ctrl_handle *ctrl;
 };
 
 /* config management */