libvlr: replace direct call of paging_expired() into a callback

libmsc internal functions shouldn't be called from within the libvlr.
Re-use the paging callback with an osmocom specific indication to
inform it to cancel paging.

Change-Id: I7a3d15e8f0fb51c6b32add2de5024fc4d599ecf0
diff --git a/include/osmocom/msc/vlr_sgs.h b/include/osmocom/msc/vlr_sgs.h
index aade5d3..7193fa5 100644
--- a/include/osmocom/msc/vlr_sgs.h
+++ b/include/osmocom/msc/vlr_sgs.h
@@ -81,7 +81,7 @@
 typedef void (*vlr_sgs_lu_response_cb_t) (struct sgs_lu_response *response);
 
 /* This callback function is called in cases where a paging request is required
- * after the LU is completed */
+ * after the LU is completed or when a paging expired */
 typedef int (*vlr_sgs_lu_paging_cb_t) (struct vlr_subscr *vsub, enum sgsap_service_ind serv_ind);
 
 /* This callback function is called to send the MM info to the UE. */