lavfi/avfilter.h: reorganize headers disposition

Group lavfi headers together, slightly improve readability.

libavfilter/avfilter.h

@@ -22,14 +22,14 @@
 #ifndef AVFILTER_AVFILTER_H
 #define AVFILTER_AVFILTER_H
 
+#include <stddef.h>
+
 #include "libavutil/avutil.h"
 #include "libavutil/log.h"
 #include "libavutil/samplefmt.h"
 #include "libavutil/pixfmt.h"
 #include "libavutil/rational.h"
 
-#include <stddef.h>
-
 #include "libavfilter/version.h"
 
 /**