nat: Add an option to set the query string to match

Allow the query string to be set. The ussd matching code
will check for this string and then forward it to the bypass.
diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index e8a4752..61f19f2 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -234,6 +234,7 @@
 
 	/* USSD messages  we want to match */
 	char *ussd_lst_name;
+	char *ussd_query;
 
 	/* statistics */
 	struct bsc_nat_statistics stats;