ext/flac/gstflacdec.*: If the stream header doesn't contain the total number of samples, search for the last flac fra...

Original commit message from CVS:
* ext/flac/gstflacdec.c: (gst_flac_calculate_crc8),
(gst_flac_dec_scan_got_frame), (gst_flac_dec_scan_for_last_block),
(gst_flac_dec_metadata_callback):
* ext/flac/gstflacdec.h:
If the stream header doesn't contain the total number of samples,
search for the last flac frame at the end of the file and calculate
the total duration from that frame's offset (fixes #337609).