mncc: Add IMSI to gsm_mncc

This allows us to do subscriber lookups based on IMSI and e.g. establish
MT calls to subscribers with no assigned extension.
diff --git a/openbsc/include/openbsc/mncc.h b/openbsc/include/openbsc/mncc.h
index c04a81f..6aa1917 100644
--- a/openbsc/include/openbsc/mncc.h
+++ b/openbsc/include/openbsc/mncc.h
@@ -193,6 +193,7 @@
 	int		more;
 	int		notify; /* 0..127 */
 	int		emergency;
+	char		imsi[16];
 
 	unsigned char	lchan_mode;
 };