[gsm0408] Make the generate_lai usable by other code

Prefix generate_lai with gsm0408 and update call sites. It will
be used from within bsc_hack.c
diff --git a/include/openbsc/gsm_04_08.h b/include/openbsc/gsm_04_08.h
index 24989b1..1513ade 100644
--- a/include/openbsc/gsm_04_08.h
+++ b/include/openbsc/gsm_04_08.h
@@ -331,6 +331,8 @@
 struct gsm_bts;
 
 int gsm0408_rcvmsg(struct msgb *msg);
+void gsm0408_generate_lai(struct gsm48_loc_area_id *lai48, u_int16_t mcc, 
+		u_int16_t mnc, u_int16_t lac);
 enum gsm_chan_t get_ctype_by_chreq(struct gsm_bts *bts, u_int8_t ra);