MINOR: mux-quic: define new unions for flow-control fields

Define two new unions in the qcc structure named 'lfctl' and 'rfctl'.
For the moment they are empty. They will be completed to store the
initial and current level for flow-control on the local and remote side.

include/haproxy/mux_quic-t.h

@@ -42,6 +42,14 @@ struct qcc {
 		} tx;
 	} strms[QCS_MAX_TYPES];
 
+	/* Flow-control related fields which are enforced on our side. */
+	struct {
+	} lfctl;
+
+	/* Flow-control related fields from the endpoint which we must respect. */
+	struct {
+	} rfctl;
+
 	struct {
 		uint64_t max_data; /* Maximum number of bytes which may be received */
 	} rx;