MINOR: connection: Add a new xprt to connection.

Simply adds XPRT_QUIC new enum to integrate QUIC transport protocol.
diff --git a/include/haproxy/connection-t.h b/include/haproxy/connection-t.h
index 71272af..0a3af44 100644
--- a/include/haproxy/connection-t.h
+++ b/include/haproxy/connection-t.h
@@ -283,6 +283,7 @@
 	XPRT_RAW = 0,
 	XPRT_SSL = 1,
 	XPRT_HANDSHAKE = 2,
+	XPRT_QUIC = 3,
 	XPRT_ENTRIES /* must be last one */
 };