MINOR: quic: Add tune.quic.retry-threshold keyword

This QUIC specific keyword may be used to set the theshold, in number of
connection openings, beyond which QUIC Retry feature will be automatically
enabled. Its default value is 100.
diff --git a/src/haproxy.c b/src/haproxy.c
index f3cf30e..1d66126 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -138,6 +138,7 @@
 #include <haproxy/uri_auth-t.h>
 #include <haproxy/vars.h>
 #include <haproxy/version.h>
+#include <haproxy/xprt_quic-t.h>
 
 
 /* array of init calls for older platforms */
@@ -204,6 +205,7 @@
 		.idle_timer = 1000, /* 1 second */
 #endif
 #ifdef USE_QUIC
+		.quic_retry_threshold = QUIC_DFLT_RETRY_THRESHOLD,
 		.quic_streams_buf = 30,
 #endif /* USE_QUIC */
 	},