mgcp-client: Rearrange internal backpointers

Let's properly store backpointers so that it is easy to access all
relation chain (pool<->pool_member<->mgcp_client).

Related: SYS#5987
Change-Id: I5ec2465075da2e3c8eabca9df60681620a7db499
diff --git a/include/osmocom/mgcp_client/mgcp_client_internal.h b/include/osmocom/mgcp_client/mgcp_client_internal.h
index 73e3de2..2eb9b4c 100644
--- a/include/osmocom/mgcp_client/mgcp_client_internal.h
+++ b/include/osmocom/mgcp_client/mgcp_client_internal.h
@@ -16,7 +16,7 @@
 	mgcp_trans_id_t next_trans_id;
 	struct llist_head responses_pending;
 	struct llist_head inuse_endpoints;
-	struct mgcp_client_pool *pool;
+	struct mgcp_client_pool_member *pool_member;
 };
 
 struct mgcp_inuse_endpoint {