avcodec/videotoolboxenc: Set default bitrate to zero

Zero is auto mode. From the doc of videotoolbox:

   The default bit rate is zero, which indicates that the video
   encoder should determine the size of compressed data.

Before the patch, the default bitrate is 200000 setting by
avcodec/options_table, which doesn't work for most of cases.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavcodec/videotoolboxenc.c

@@ -2933,6 +2933,7 @@ static const AVOption h264_options[] = {
 };
 
 static const FFCodecDefault vt_defaults[] = {
+        {"b",    "0"},
         {"qmin", "-1"},
         {"qmax", "-1"},
         {NULL},