osmux: send osmux stats in MGCP DLCX responses

This allows us to know what number of messages and bytes has been
received per active osmux endpoint.

Note that an Osmux message is composed of several chunks. Each chunk
contains an osmux header plus several voice data frames.

 P: PS=385, OS=11188, PR=195, OR=5655, PL=0, JI=49
 X-Osmo-CP: EC TIS=0, TOS=0, TIR=0, TOR=0
 X-Osmux-ST: CR=51, BR=3129

The new 'X-Osmux-ST:' notifies the received chunks and bytes.
diff --git a/openbsc/include/openbsc/mgcp_internal.h b/openbsc/include/openbsc/mgcp_internal.h
index 34c3d97..3bccb39 100644
--- a/openbsc/include/openbsc/mgcp_internal.h
+++ b/openbsc/include/openbsc/mgcp_internal.h
@@ -190,6 +190,11 @@
 		struct osmux_in_handle *in;
 		/* handle to unbatch messages */
 		struct osmux_out_handle out;
+		/* statistics */
+		struct {
+			uint32_t chunks;
+			uint32_t octets;
+		} stats;
 	} osmux;
 };