MINOR: quic: Replace the RX list of packet by a thread safety one.

This list is shared between the I/O dgram handler and the task responsible
for processing the QUIC packets.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 6a0f9bf..cb052b4 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -3399,7 +3399,7 @@
 	}
 	else if (!found_conn) {
 		/* Enqueue this packet. */
-		LIST_APPEND(&l->rx.qpkts, &pkt->rx_list);
+		MT_LIST_APPEND(&l->rx.pkts, &pkt->rx_list);
 		/* Try to accept a new connection. */
 		listener_accept(l);
 	}