org-num-skip-tags: Avoid using function that is not pre-loaded in autoloads

* lisp/org-num.el (org-num-skip-tags): Make sure that function used
for :safe `defcustom' slot does not require functions that are not
defined in org-loaddefs.el.  This is because `org-num-skip-tags' is
autoloaded and cannot rely upon requires in org-num.el.  Instead, it
may only use pre-loaded functions and other autoloaded Org mode
functions.

Reported-by: Eli Zaretskii <eliz@gnu.org>
Link: https://yhetil.org/emacs-devel/868qzd9hjg.fsf@gnu.org/

lisp/org-compat.el

@@ -251,6 +251,17 @@ removed."
                     default)))
      ": ")))
 
+(if (fboundp 'list-of-strings-p)
+    (defalias 'org-list-of-strings-p #'list-of-strings-p)
+  ;; From Emacs subr.el.
+;;;###autoload
+  (defun org-list-of-strings-p (object)
+    "Return t if OBJECT is nil or a list of strings."
+    (declare (pure t) (side-effect-free error-free))
+    (while (and (consp object) (stringp (car object)))
+      (setq object (cdr object)))
+    (null object)))
+
 
 ;;; Emacs < 27.1 compatibility
 

lisp/org-num.el

@@ -144,7 +144,7 @@ control tag inheritance."
   :group 'org-appearance
   :package-version '(Org . "9.3")
   :type '(repeat (string :tag "Tag"))
-  :safe (lambda (val) (and (listp val) (cl-every #'stringp val))))
+  :safe #'org-list-of-strings-p)
 
 ;;;###autoload
 (defcustom org-num-skip-unnumbered nil