Class name is used in show_help_children(avfilter_get_class(),...)
to prompt the available filters.

$ ffmpeg -h full

Before:
qsvscale AVOptions:

After:
scale_qsv AVOptions:
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Zhong Li <zhongli_dev@126.com>