qtdemux: opus: set entry as sampled

... otherwise streams with constant size samples defined with a single sample_size for all samples in the stsz box fall in the category chunks_are_samples in qtdemux_stbl_init, overriding the actual sample count.

FOURCC_soun would set this automatically for compression_id == 0xfffe, however compression_id is read from the Audio Sample Entry box at an offset marked as "pre-defined" in some version of the spec and set to 0 both by GStreamer and FFmpeg for opus streams.

Considering the stream sampled flag is set explicitely by other fourcc variants, doing so for opus seems consistent.