org.el: Specify minimum Emacs version in Package-Requires header

* lisp/org.el: Specify minimum Emacs version in Package-Requires
header.

Suggested-by: Stefan Monnier <monnier@iro.umontreal.ca>
Link: https://orgmode.org/list/jwvv9a01l1o.fsf-monnier+emacs@gnu.org

lisp/org.el

@@ -7,6 +7,7 @@
 ;; Maintainer: Bastien Guerry <bzg@gnu.org>
 ;; Keywords: outlines, hypermedia, calendar, wp
 ;; Homepage: https://orgmode.org
+;; Package-Requires: ((emacs "24.3"))
 
 ;; Version: 9.5-dev