avcodec/libaomenc: increase the default bitrate

aom_codec_enc_config_default() sets it to 256kbps, so don't replace
it with 200kbps.

See https://bugs.chromium.org/p/aomedia/issues/detail?id=2219

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/libaomenc.c

@@ -986,6 +986,7 @@ static const AVOption options[] = {
 };
 
 static const AVCodecDefault defaults[] = {
+    { "b",          "256*1000" },
     { "qmin",             "-1" },
     { "qmax",             "-1" },
     { "g",                "-1" },