riff: Add DAVC fourcc.

This fourcc is used by the "mpegable AVC" codec and files encoded with
this codec decode correctly with our H.264 decoder.

libavformat/riff.c

@@ -33,6 +33,7 @@ const AVCodecTag ff_codec_bmp_tags[] = {
     { CODEC_ID_H264,         MKTAG('X', '2', '6', '4') },
     { CODEC_ID_H264,         MKTAG('x', '2', '6', '4') },
     { CODEC_ID_H264,         MKTAG('a', 'v', 'c', '1') },
+    { CODEC_ID_H264,         MKTAG('D', 'A', 'V', 'C') },
     { CODEC_ID_H264,         MKTAG('V', 'S', 'S', 'H') },
     { CODEC_ID_H263,         MKTAG('H', '2', '6', '3') },
     { CODEC_ID_H263,         MKTAG('X', '2', '6', '3') },