add new function gsm_bts_by_lac() to search for BTS based on location area

diff --git a/include/openbsc/gsm_data.h b/include/openbsc/gsm_data.h
index 4da9e3a..0e3a380 100644
--- a/include/openbsc/gsm_data.h
+++ b/include/openbsc/gsm_data.h
@@ -349,6 +349,8 @@
 		   u_int8_t e1_ts, u_int8_t e1_ts_ss);
 enum gsm_bts_type parse_btstype(char *arg);
 char *btstype2str(enum gsm_bts_type type);
+struct gsm_bts *gsm_bts_by_lac(struct gsm_network *net, unsigned int lac,
+				struct gsm_bts *start_bts);
 
 static inline int is_ipaccess_bts(struct gsm_bts *bts)
 {