MINOR: quic: Attach QUIC mux connection objet to QUIC connection.

This add a qcc struct for QUIC mux/demux connection layer to quic_conn struct
at low level connection layer.
diff --git a/include/haproxy/xprt_quic-t.h b/include/haproxy/xprt_quic-t.h
index ed99465..d55c36b 100644
--- a/include/haproxy/xprt_quic-t.h
+++ b/include/haproxy/xprt_quic-t.h
@@ -628,6 +628,8 @@
 	struct quic_path paths[1];
 	struct quic_path *path;
 
+	/* MUX */
+	struct qcc *qcc;
 	struct task *timer_task;
 	unsigned int timer;
 };