avcodec/vda: define av_vda_default_init2 when CONFIG_H264_VDA_HWACCEL equ 0

avcodec/vda: define av_vda_default_init2 when CONFIG_H264_VDA_HWACCEL equ 0

on OSX:
../configure --disable-everything --enable-demuxer=hls make
error message: Undefined symbols for architecture x86_64:
"_av_vda_default_init2", referenced from:_videotoolbox_init in
ffmpeg_videotoolbox.o
so add av_vda_default_init2 when CONFIG_H264_VDA_HWACCEL=0

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Reviewed-by: Xidorn Quan <quanxunzhen@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>