gst/avi/gstavimux.*: Add 8 bytes to current streamheader to make for a complete one and to make more players happy.  ...

Original commit message from CVS:
* gst/avi/gstavimux.c: (gst_avi_mux_riff_get_avi_header):
* gst/avi/gstavimux.h:
Add 8 bytes to current streamheader to make for a complete one
and to make more players happy.  Fixes #519460.