MINOR: stream: provide a new stream creation function for connections

The purpose will be to create new streams for a given connection so
that we can later abstract this from a mux.
diff --git a/include/proto/stream.h b/include/proto/stream.h
index 44fc8be..00f452c 100644
--- a/include/proto/stream.h
+++ b/include/proto/stream.h
@@ -36,6 +36,7 @@
 extern struct data_cb sess_conn_cb;
 
 struct stream *stream_new(struct session *sess, enum obj_type *origin);
+int stream_create_from_conn(struct connection *conn);
 
 /* perform minimal intializations, report 0 in case of error, 1 if OK. */
 int init_stream();