Make diredp-dir-heading inherit face from dired-header.

`diredp-dir-heading` tries to inherit face attributes from the face `heading`.  `heading` is not a defined face in a standard Emacs installation or this theme.

`dired-header` should always exist as `dired` is a part of GNU Emacs.

This problem presented itself when I was attempting to successfully call `org-html-htmlize-generate-css`.  An error is thrown by `face-attribute` which chokes on the undefined face and halts execution of the CSS generation function.  `htmlize.el` could do a better job of handling invalid faces.  I've searched for that repository to suggest a patch without any luck.  Regardless, inheriting from a non-standard undefined face is probably not a great idea.