BUILD: quic: fix build warning when threads are disabled

Commit e83f937cc ("MEDIUM: quic: use a global CID trees list") uses a
local variable "tree" used only for locks, but when threads are disabled
it spews a warning about this unused variable.
diff --git a/include/haproxy/quic_conn.h b/include/haproxy/quic_conn.h
index ef1ccf4..4640e08 100644
--- a/include/haproxy/quic_conn.h
+++ b/include/haproxy/quic_conn.h
@@ -165,7 +165,7 @@
 static inline void quic_cid_delete(struct quic_connection_id *conn_id)
 {
 	const uchar idx = quic_cid_tree_idx(&conn_id->cid);
-	struct quic_cid_tree *tree = &quic_cid_trees[idx];
+	struct quic_cid_tree __maybe_unused *tree = &quic_cid_trees[idx];
 
 	HA_RWLOCK_WRLOCK(QC_CID_LOCK, &tree->lock);
 	ebmb_delete(&conn_id->node);