[Wp] avoid using Stdlib__* names, incompatible with OCaml 4.13

src/plugins/wp/cil2cfg.ml

@@ -1395,7 +1395,7 @@ struct
   type dot = {
     name: string;
     chan: out_channel;
-    fmt : Stdlib__format.formatter;
+    fmt : Format.formatter;
   }
 
   let create kf =