CLEANUP: connection: Use `VAR_ARRAY` in `struct tlv` definition

This is for consistency with `struct tlv_ssl`.
diff --git a/include/haproxy/connection-t.h b/include/haproxy/connection-t.h
index 9c2af7b..0bcceac 100644
--- a/include/haproxy/connection-t.h
+++ b/include/haproxy/connection-t.h
@@ -649,7 +649,7 @@
 	uint8_t type;
 	uint8_t length_hi;
 	uint8_t length_lo;
-	uint8_t value[0];
+	uint8_t value[VAR_ARRAY];
 }__attribute__((packed));
 
 struct tlv_ssl {