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>

libavcodec/vda.c

@@ -73,6 +73,11 @@ int av_vda_default_init(AVCodecContext *avctx)
     return AVERROR(ENOSYS);
 }
 
+int av_vda_default_init2(AVCodecContext *avctx, AVVDAContext *vdactx)
+{
+    return AVERROR(ENOSYS);
+}
+
 void av_vda_default_free(AVCodecContext *ctx)
 {
 }