bsc: Move the finding of a MSC into the filter code

For responding to paging on the right link we will need to
figure out if the msg is a paging response.
diff --git a/openbsc/include/openbsc/osmo_bsc.h b/openbsc/include/openbsc/osmo_bsc.h
index 0e904c9..bf582b2 100644
--- a/openbsc/include/openbsc/osmo_bsc.h
+++ b/openbsc/include/openbsc/osmo_bsc.h
@@ -36,6 +36,7 @@
 			      struct osmo_msc_data *msc);
 int bsc_delete_connection(struct osmo_bsc_sccp_con *sccp);
 
+struct osmo_msc_data *bsc_find_msc(struct gsm_subscriber_connection *conn, struct msgb *);
 int bsc_scan_bts_msg(struct gsm_subscriber_connection *conn, struct msgb *msg);
 int bsc_scan_msc_msg(struct gsm_subscriber_connection *conn, struct msgb *msg);