mscsplit: add gsm_network backpointer to gsm_subscriber_connection

We want to be able to use a network backpointer without having to go through a
gsm_bts struct.

This commit adds the network pointer, the subsequent commit applies direct
access to the network structure from gsm_subscriber_connection.

Change-Id: If8870972f1b3e333c2a4cce97cdc95bdee0382a1
diff --git a/openbsc/src/libbsc/bsc_api.c b/openbsc/src/libbsc/bsc_api.c
index f42598a..02751f4 100644
--- a/openbsc/src/libbsc/bsc_api.c
+++ b/openbsc/src/libbsc/bsc_api.c
@@ -247,6 +247,7 @@
 	if (!conn)
 		return NULL;
 
+	conn->network = net;
 	conn->lchan = lchan;
 	conn->bts = lchan->ts->trx->bts;
 	lchan->conn = conn;