drawutils: fix copyright

git blame:
    423 Nicolas George
     86 Stefano Sabatini
     19 Michael Niedermayer
      1 Reimar Döffinger

Initial commit:
commit bcfd9e821b
Author: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Date:   Sun Feb 20 23:42:17 2011 +0100

    lavfi: add drawutils

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

libavfilter/drawutils.c

@@ -1,4 +1,7 @@
 /*
+ * Copyright 2011 Stefano Sabatini <stefano.sabatini-lala poste it>
+ * Copyright 2012 Nicolas George <nicolas.george normalesup org>
+ *
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or