gsm_data: make cgi_for_msc available for other modules

The function cgi_for_msc() provides an easy way to get a cell global id
for an msc/bts combination. This function is currently statically
defined in gsm_08_08.c. Lets move it to gsm_data.c and make it publicly
available.

Change-Id: I301fac6e83a429ae59b5c586aa283ad7ba54053d
Related: OS#3645
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 7897fea..1be9ae8 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -1614,4 +1614,6 @@
 
 int bts_count_free_ts(struct gsm_bts *bts, enum gsm_phys_chan_config pchan);
 
+struct osmo_cell_global_id *cgi_for_msc(struct bsc_msc_data *msc, struct gsm_bts *bts);
+
 #endif /* _GSM_DATA_H */