refactor paging: introduce bsc_paging_start()

Allow starting a paging from elsewhere than a BSSMAP Paging Request. For
upcoming Location Services (LCS), a BSSLAP TA Request from the SMLC may require
triggering a Paging.

Change-Id: Iaff91584699d163bd1963927280ff3a8ddd43073
diff --git a/tests/handover/handover_test.c b/tests/handover/handover_test.c
index ba0ec21..bdf926d 100644
--- a/tests/handover/handover_test.c
+++ b/tests/handover/handover_test.c
@@ -48,6 +48,7 @@
 #include <osmocom/bsc/handover_fsm.h>
 #include <osmocom/bsc/bsc_msc_data.h>
 #include <osmocom/bsc/bts.h>
+#include <osmocom/bsc/paging.h>
 
 void *ctx;
 
@@ -1798,6 +1799,8 @@
 void bsc_cipher_mode_compl(struct gsm_subscriber_connection *conn, struct msgb *msg, uint8_t chosen_encr) {}
 int bsc_compl_l3(struct gsm_lchan *lchan, struct msgb *msg, uint16_t chosen_channel)
 { return 0; }
+int bsc_paging_start(struct bsc_paging_params *params)
+{ return 0; }
 void bsc_dtap(struct gsm_subscriber_connection *conn, uint8_t link_id, struct msgb *msg) {}
 void bsc_assign_compl(struct gsm_subscriber_connection *conn, uint8_t rr_cause) {}
 void bsc_cm_update(struct gsm_subscriber_connection *conn,