Move global apn_list inside struct sgsn_instance

This way apns are managed by the lifcycle of the main global struct
sgsn_instance automatically.

Change-Id: I8cc8e540cfb64d0f130e9c0aaedf7b0835f8fe16
diff --git a/include/osmocom/sgsn/sgsn.h b/include/osmocom/sgsn/sgsn.h
index 7e78eee..0bc88a9 100644
--- a/include/osmocom/sgsn/sgsn.h
+++ b/include/osmocom/sgsn/sgsn.h
@@ -151,6 +151,7 @@
 
 	struct rate_ctr_group *rate_ctrs;
 
+	struct llist_head apn_list; /* list of struct sgsn_apn_ctx */
 	struct llist_head ggsn_list; /* list of struct sgsn_ggsn_ctx */
 	struct llist_head mme_list; /* list of struct sgsn_mme_ctx */