lavfi/zscale: make use of AVFILTER_DEFINE_CLASS

use AVFILTER_DEFINE_CLASS for defining the filter classes
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavfilter/vf_zscale.c

@@ -859,13 +859,7 @@ static const AVOption zscale_options[] = {
     { NULL }
 };
 
-static const AVClass zscale_class = {
-    .class_name       = "zscale",
-    .item_name        = av_default_item_name,
-    .option           = zscale_options,
-    .version          = LIBAVUTIL_VERSION_INT,
-    .category         = AV_CLASS_CATEGORY_FILTER,
-};
+AVFILTER_DEFINE_CLASS(zscale);
 
 static const AVFilterPad avfilter_vf_zscale_inputs[] = {
     {