Revert "Remove leftover from previous electric-layout-rules API"

This is a backward-incompatible change, and removing it isn't really
necessary.

This reverts commit 942dad2b51.

lisp/electric.el

@@ -414,6 +414,7 @@ If multiple rules match, only first one is executed.")
                ;; Not in a string or comment.
                (not (nth 8 (save-excursion (syntax-ppss pos)))))
       (goto-char pos)
+      (when (functionp rule) (setq rule (funcall rule)))
       (dolist (sym (if (symbolp rule) (list rule) rule))
         (let* ((nl-after
                 (lambda ()