gst/matroska/matroska-demux.c: Make mpeg2 aac audio work: create artificial private codec data chunk which faad2 seem...

Original commit message from CVS:
* gst/matroska/matroska-demux.c: (gst_matroska_demux_audio_caps),
(gst_matroska_demux_plugin_init):
Make mpeg2 aac audio work: create artificial private codec data
chunk which faad2 seems to require, just as we do for mpeg4 aac.
Also call gst_riff_init(). Partially fixes #338767.