lavfi/buffer.c: fix copyright

git blame:
     75 Anton Khirnov
     72 Michael Niedermayer
     39 Stefano Sabatini
     23 S.N. Hemanth Meenakshisundaram
      9 Vitor Sessak
      6 Robert Nagy
      2 Diego Biurrun
      1 Andrey Utkin

Note:
commit ab165047
Author: Anton Khirnov <anton@khirnov.net>
Date:   Fri Apr 27 17:27:40 2012 +0200

    lavfi: add a function for copying properties from AVFilterBufferRef->AVFrame

    Based on a commit by Stefano Sabatini <stefano.sabatini-lala@poste.it>

commit 4a1ac8c4
Author: Anton Khirnov <anton@khirnov.net>
Date:   Thu May 10 07:58:11 2012 +0200

    lavfi: move buffer management function to a separate file.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/buffer.c

 /*
+ * Copyright Stefano Sabatini <stefasab gmail com>
+ * Copyright Anton Khirnov <anton khirnov net>
+ * Copyright Michael Niedermayer <michaelni gmx at>
+ *
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or