Constify msc_subscr_conn_is_accepted() parameter

Related: OS#2864
Change-Id: I8af50a8847a5b438cf2ef660399d4c8bbac86a71
diff --git a/include/osmocom/msc/osmo_msc.h b/include/osmocom/msc/osmo_msc.h
index bf455c3..2fa8c46 100644
--- a/include/osmocom/msc/osmo_msc.h
+++ b/include/osmocom/msc/osmo_msc.h
@@ -79,7 +79,7 @@
 		     uint8_t cause, uint8_t *rr_cause);
 
 void msc_subscr_conn_init(void);
-bool msc_subscr_conn_is_accepted(struct gsm_subscriber_connection *conn);
+bool msc_subscr_conn_is_accepted(const struct gsm_subscriber_connection *conn);
 void msc_subscr_conn_communicating(struct gsm_subscriber_connection *conn);
 void msc_subscr_conn_close(struct gsm_subscriber_connection *conn,
 			   uint32_t cause);