mkimage: Fix missing free() in show_valid_options()

The allocated memory should be freed. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 150963)
Reviewed-by: Tom Rini <trini@konsulko.com>

tools/mkimage.c

@@ -64,6 +64,7 @@ static int show_valid_options(enum ih_category category)
 			genimg_get_cat_name(category, item));
 	}
 	fprintf(stderr, "\n");
+	free(order);
 
 	return 0;
 }