fix(lib): autoload format-spec on 27.x

This function wasn't autoloaded until 28, causing void-function errors
if it's used.

core/core-lib.el

@@ -781,6 +781,9 @@ testing advice (when combined with `rotate-text').
 ;;; Backports
 
 (unless EMACS28+
+  ;; `format-spec' wasn't autoloaded until 28
+  (autoload #'format-spec "format-spec")
+
   (defun ensure-list (object)
     "Return OBJECT as a list.
 If OBJECT is already a list, return OBJECT itself.  If it's