x86/blockdsp: restore author attribution

See commits
649c00c96d
5fecfb7d58
73b02e2460

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/x86/blockdsp.asm

@@ -1,6 +1,8 @@
 ;******************************************************************************
-;*
 ;* SIMD-optimized clear block functions
+;* Copyright (c) 2002 Michael Niedermayer
+;* Copyright (c) 2008 Loren Merritt
+;* Copyright (c) 2009 Jason Garrett-Glaser
 ;*
 ;* This file is part of FFmpeg.
 ;*