Fix ord unused-type-declaration warning disable pre-OCaml 4.08 (PR #260)

src_plugins/ord/ppx_deriving_ord.cppo.ml

@@ -194,7 +194,7 @@ let str_of_type ~options ~path ({ ptype_loc = loc } as type_decl) =
      conflicts with a Stdlib type from Ppx_deriving_runtime (e.g. bool in test).
      In that case we must refer to the type being declared, not the one opened by Ppx_deriving_runtime. *)
   let helper_type =
-    Type.mk ~loc ~attrs:[Ppx_deriving.attr_warning [%expr "-unused-type-declaration"]]
+    Type.mk ~loc ~attrs:[Ppx_deriving.attr_warning [%expr "-34"]] (* unused-type-declaration *)
       ~params:type_decl.ptype_params
       ~manifest:(Ppx_deriving.core_type_of_type_decl type_decl)
       (mkloc "t" loc)