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

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.