opus_parser: make ParseContext the first element in OpusParseContext

ff_parse_close expects priv_data to be the ParseContext directly and
thus doesn't work if it isn't at the beginning of OpusParseContext.

Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>