vf_fps: fix copyright

Only commit:
commit 54c5dd89e3
Author: Anton Khirnov <anton@khirnov.net>
Date:   Wed May 9 14:08:21 2012 +0200

    lavfi: Add fps filter.

    Partially based on a patch by Robert Nagy <ronag89@gmail.com>

also see [FFmpeg-devel] [PATCH 07/10] vf_fps: fix copyright

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

libavfilter/vf_fps.c

@@ -1,4 +1,8 @@
 /*
+ * Copyright 2007 Bobby Bingham
+ * Copyright 2012 Robert Nagy <ronag89 gmail com>
+ * Copyright 2012 Anton Khirnov <anton khirnov net>
+ *
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or