lavfi/tonemap: 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_tonemap.c

@@ -287,13 +287,7 @@ static const AVOption tonemap_options[] = {
     { NULL }
 };
 
-static const AVClass tonemap_class = {
-    .class_name       = "tonemap",
-    .item_name        = av_default_item_name,
-    .option           = tonemap_options,
-    .version          = LIBAVUTIL_VERSION_INT,
-    .category         = AV_CLASS_CATEGORY_FILTER,
-};
+AVFILTER_DEFINE_CLASS(tonemap);
 
 static const AVFilterPad tonemap_inputs[] = {
     {