Revert "* lisp/image-dired.el: Remove unnecessary 'declare-function'."

This reverts commit e0261d4.

This commit gives warnings on builds --without-x.
Problem pointed out by Glenn Morris <rgm@gnu.org> in:
https://lists.gnu.org/r/emacs-devel/2021-11/msg01278.html

lisp/image-dired.el

@@ -1739,6 +1739,8 @@ Note that n, p and <down> and <up> will be hijacked and bound to
 `image-dired-dired-next-line' and `image-dired-dired-previous-line'."
   :keymap image-dired-minor-mode-map)
 
+(declare-function clear-image-cache "image.c" (&optional filter))
+
 (defun image-dired-create-thumbs (&optional arg)
   "Create thumbnail images for all marked files in Dired.
 With prefix argument ARG, create thumbnails even if they already exist