af_null: fix copyright

Authors according to blame are:
     16 S.N. Hemanth Meenakshisundaram
      4 Stefano Sabatini
      3 Anton Khirnov
      2 Reimar Döffinger

Initial commit was:
	commit 99046339fc
	Author: S.N. Hemanth Meenakshisundaram <smeenaks@ucsd.edu>
	Date:   Fri Aug 20 23:16:51 2010 +0000

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/af_anull.c

@@ -1,4 +1,5 @@
 /*
+ * Copyright (c) 2010 S.N. Hemanth Meenakshisundaram <smeenaks@ucsd.edu>
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or