Bugfix 20618: need for parenthesis around (fun x -> ) in ocaml extraction

examples/bts/20618.mlw

+module Bug
+  use HighOrd
+  
+  type foo = { a: unit -> unit ; b : unit }
+
+  constant bar : foo = {
+    a = \_. () ;
+    b = ()
+  }
+end

src/whyml/mlw_ocaml.ml

@@ -950,7 +950,7 @@ module Print = struct
         fprintf fmt (protect_on paren "@[<hov 2>%a@ %a@]")
           (print_expr info) e (print_list space (print_expr_p info)) el
     | Efun (vl, e1) ->
-        fprintf fmt (protect_on paren "@[<hov 2>fun %a ->@ %a@]")
+        fprintf fmt (protect_on paren "@[<hov 2>(fun %a ->@ %a)@]")
           (print_list space (print_vs_arg info)) vl (print_expr info) e1;
         forget_vars vl
     | Econstr (c, []) ->