riff: add "YUV8" FourCC

sample:

http://samples.mplayerhq.hu/V-codecs/yuv8/MAILTEST.AVI

Reviewed-by: compn <tempn@twmi.rr.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/riff.c

@@ -262,6 +262,7 @@ const AVCodecTag ff_codec_bmp_tags[] = {
     { CODEC_ID_VC1IMAGE,     MKTAG('W', 'V', 'P', '2') },
     { CODEC_ID_LOCO,         MKTAG('L', 'O', 'C', 'O') },
     { CODEC_ID_WNV1,         MKTAG('W', 'N', 'V', '1') },
+    { CODEC_ID_WNV1,         MKTAG('Y', 'U', 'V', '8') },
     { CODEC_ID_AASC,         MKTAG('A', 'A', 'S', '4') },
     { CODEC_ID_AASC,         MKTAG('A', 'A', 'S', 'C') },
     { CODEC_ID_INDEO2,       MKTAG('R', 'T', '2', '1') },