ext/flac/gstflacdec.c: Add support for segment seeks (fixes #338290). Also demote some recurring debug message from D...

Original commit message from CVS:
* ext/flac/gstflacdec.c: (gst_flac_dec_loop),
(gst_flac_dec_handle_seek_event):
Add support for segment seeks (fixes #338290). Also demote
some recurring debug message from DEBUG to LOG level.