bts: Store the bcch_change_mark in the bts structure

Store the BCCH change mark inside the BTS structure. This will
allow us increment the number and re-generate the SIs.

Related: SYS#739
diff --git a/openbsc/src/libbsc/system_information.c b/openbsc/src/libbsc/system_information.c
index a3deefc..97827f2 100644
--- a/openbsc/src/libbsc/system_information.c
+++ b/openbsc/src/libbsc/system_information.c
@@ -792,6 +792,9 @@
 	si13_default.no_pbcch.rac = bts->gprs.rac;
 	si13_default.no_pbcch.net_ctrl_ord = bts->gprs.net_ctrl_ord;
 
+	/* Information about the other SIs */
+	si13_default.bcch_change_mark = bts->bcch_change_mark;
+
 	ret = rest_octets_si13(si13->rest_octets, &si13_default);
 	if (ret < 0)
 		return ret;