riff: add SMV2 FourCC for H264

sample:

http://samples.mplayerhq.hu/V-codecs/smv2.aviSigned-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/riff.c

@@ -39,6 +39,7 @@ const AVCodecTag ff_codec_bmp_tags[] = {
     { AV_CODEC_ID_H264,         MKTAG('x', '2', '6', '4') },
     { AV_CODEC_ID_H264,         MKTAG('a', 'v', 'c', '1') },
     { AV_CODEC_ID_H264,         MKTAG('D', 'A', 'V', 'C') },
+    { AV_CODEC_ID_H264,         MKTAG('S', 'M', 'V', '2') },
     { AV_CODEC_ID_H264,         MKTAG('V', 'S', 'S', 'H') },
     { AV_CODEC_ID_H264,         MKTAG('Q', '2', '6', '4') }, /* QNAP surveillance system */
     { AV_CODEC_ID_H263,         MKTAG('H', '2', '6', '3') },