libosmo-mgcp-client: extend the mgcp_client for MGW pooling

At the moment the MGCP Client only supports one MGW per application.
Depending on the requirements of the application one MGW might not offer
the performance needed. Lets add support for an MGCP Client pool that is
backward compatible to existing applications.

Change-Id: Icaaba0e470e916eefddfee750b83f5f65291a6b0
Related: SYS#5091
diff --git a/include/osmocom/mgcp_client/mgcp_client_internal.h b/include/osmocom/mgcp_client/mgcp_client_internal.h
index 44b9022..73e3de2 100644
--- a/include/osmocom/mgcp_client/mgcp_client_internal.h
+++ b/include/osmocom/mgcp_client/mgcp_client_internal.h
@@ -16,6 +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_inuse_endpoint {