build: make sure 'dune build' works in every dir (with a dune file)

test/toy-example-sum/state.ml

@@ -8,7 +8,7 @@ type t = {
   res:int
 }
 
-let (to_string: (t -> string)) =
+let (_to_string: (t -> string)) =
   fun s ->
     Printf.sprintf "input=%d sub=%d res=%d"  s.input s.sub s.res