[wp] Use Extlib.mkdir for session directory

src/plugins/wp/wp_parameters.ml

@@ -1051,7 +1051,7 @@ let make_output_dir dir =
   else
     begin
       try
-        Unix.mkdir dir 0o770 ;
+        Extlib.mkdir ~parents:true dir 0o770 ;
         debug ~dkey "Created output directory '%s'" dir
       with Unix.Unix_error (err,_,_) ->
         let msg = Unix.error_message err in