Sparse fix for builtin-diff

You gotta love sparse:

builtin-diff.c:88:4: error: Just how const do you want this type to be?

Signed-off-by: Peter Hagervall <hager@cs.umu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-diff.c

@@ -84,7 +84,6 @@ static void stuff_change(struct diff_options *opt,
 
 	if (opt->reverse_diff) {
 		unsigned tmp;
-		const
 		const unsigned char *tmp_u;
 		const char *tmp_c;
 		tmp = old_mode; old_mode = new_mode; new_mode = tmp;