lavfi: use designated initializers in avfilter_class

libavfilter/avfilter.c

@@ -320,10 +320,9 @@ static const char *filter_name(void *p)
 }
 
 static const AVClass avfilter_class = {
-    "AVFilter",
-    filter_name,
-    NULL,
-    LIBAVUTIL_VERSION_INT,
+    .class_name = "AVFilter",
+    .item_name  = filter_name,
+    .version    = LIBAVUTIL_VERSION_INT,
 };
 
 int avfilter_open(AVFilterContext **filter_ctx, AVFilter *filter, const char *inst_name)