lavc/dsputil: add missing vsad8 and vsse8 functions

the mpeg encoder would try to use them if vsad or vsse were selected for frame_skip_cmp
and frame_skip_threshold or frame_skip_factor were set to values != 0

example: "ffmpeg -i INPUT -c:v mpeg2video -skipcmp vsad -skip_threshold 1 -f null -"

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