matroska: Add V_VP8

Patch by Google

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@23192 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/matroska.c

@@ -76,6 +76,7 @@ const CodecTags ff_mkv_codec_tags[]={
     {"V_SNOW"           , CODEC_ID_SNOW},
     {"V_THEORA"         , CODEC_ID_THEORA},
     {"V_UNCOMPRESSED"   , CODEC_ID_RAWVIDEO},
+    {"V_VP8"            , CODEC_ID_VP8},
 
     {""                 , CODEC_ID_NONE}
 };