lavf/dashenc: set FLAC manifest codec string to "flac"

Internally in ISOBMFF the FLAC-in-ISOBMFF draft uses "fLaC"
as the identifier for FLACSampleEntry, and there seems to be no
MPEG-DASH specification for the in-manifest identifier for FLAC.

After testing the browsers' implementations, it seems like all of
the major browser vendors have decided to utilize the MIME type for
FLAC ("audio/flac") as the identifier. This change set leads to
that string being utilized for FLAC streams instead of the sample
entry identifier ("fLaC"), which is the default behavior.

Verified by auri_ on IRC to play with the major browsers.

libavformat/dashenc.c

@@ -143,6 +143,7 @@ static struct codec_string {
     { AV_CODEC_ID_VP9, "vp9" },
     { AV_CODEC_ID_VORBIS, "vorbis" },
     { AV_CODEC_ID_OPUS, "opus" },
+    { AV_CODEC_ID_FLAC, "flac" },
     { 0, NULL }
 };