[system_information] fix BCCH list generation for GSM900

We have to make sure to skip the global list header
diff --git a/openbsc/src/system_information.c b/openbsc/src/system_information.c
index d6d166c..30d15ac 100644
--- a/openbsc/src/system_information.c
+++ b/openbsc/src/system_information.c
@@ -136,6 +136,8 @@
 	if (bts->band == GSM_BAND_900) {
 		chan_list[0] = 0;
 		llist_for_each_entry(cur_bts, &bts->list, list) {
+			if (&cur_bts->list == &bts->network->bts_list)
+				continue;
 			trx = cur_bts->c0;
 			rc = cchan_list_bm0_set_arfcn(chan_list, trx->arfcn);
 			if (rc < 0)