libmat2: audio: add the audio/x-flac mime type

The FLAC parser looks for the 'audio/flac' mime type, but Fedora
defines 'audio/x-flac' in /etc/mime.types for FLAC files. Add this mime
type to the audio parser.

Fixes #36.

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>