Update runtime files

runtime/indent/sdl.vim

@@ -1,7 +1,7 @@
 " Vim indent file
 " Language:	SDL
 " Maintainer:	Michael Piefel <entwurf@piefel.de>
-" Last Change:	10 December 2011
+" Last Change:	2021 Oct 03
 
 " Shamelessly stolen from the Vim-Script indent file
 
@@ -14,6 +14,8 @@ let b:did_indent = 1
 setlocal indentexpr=GetSDLIndent()
 setlocal indentkeys+==~end,=~state,*<Return>
 
+let b:undo_indent = "setl inde< indk<"
+
 " Only define the function once.
 if exists("*GetSDLIndent")
 "  finish