avformat/apetag: fix flag value to signal footer presence

avformat/apetag: fix flag value to signal footer presence

According to the spec[1], a value of 0 means the footer is present and a value
of 1 means it's absent, the exact opposite of header presence flag where 1
means present and 0 absent.
The reason for this is compatibility with APEv1 tags, where there's no header,
footer presence was mandatory for all files, and the flags field was a zeroed
reserved field.

[1] http://wiki.hydrogenaud.io/index.php?title=Ape_Tags_Flags

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>