avconv: Use the mpeg12 private option scan_offset

Introduced in aed790070486b1b01b48106310d9d0ca1730e459

Bug-Id: debian/773055
CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Anton Khirnov <anton@khirnov.net>