refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/eval/vars.c

@@ -413,7 +413,7 @@ void list_hashtable_vars(hashtab_T *ht, const char *prefix, int empty, int *firs
       // apply :filter /pat/ to variable name
       xstrlcpy(buf, prefix, IOSIZE);
       xstrlcat(buf, (char *)di->di_key, IOSIZE);
-      if (message_filtered((char_u *)buf)) {
+      if (message_filtered(buf)) {
         continue;
       }