runtime: Set b:undo_ftplugin where missing (#12943)

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/art.vim

@@ -3,6 +3,7 @@
 " Maintainer:    Dorai Sitaram <ds26@gte.com>
 " URL:		 http://www.ccs.neu.edu/~dorai/vimplugins/vimplugins.html
 " Last Change:   Apr 2, 2003
+"                2023 Aug 28 by Vim Project (undo_ftplugin)
 
 " Only do this when not done yet for this buffer
 if exists("b:did_ftplugin")
@@ -13,3 +14,5 @@ run ftplugin/lisp.vim
 
 setl lw-=if
 setl lw+=def-art-fun,deffacts,defglobal,defrule,defschema,for,schema,while
+
+let b:undo_ftplugin ..= " | setl lw<"