avfilter: use designated initializer for class

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/avfilter.c

@@ -373,10 +373,9 @@ 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)