xtea.h: include author attribution

Reviewed-by: Clément Bœsch <ubitux@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/xtea.h

 /*
  * A 32-bit implementation of the XTEA algorithm
+ * Copyright (c) 2012 Samuel Pitoiset
  *
  * This file is part of FFmpeg.
  *