Add :version to org-gnus.el option.

lisp/org-gnus.el

@@ -62,6 +62,7 @@ configured) IMAP servers don't support this operation quickly.
 So if following a link to a Gnus article takes ages, try setting
 this variable to `t'."
   :group 'org-link-store
+  :version "24.1"
   :type 'boolean)