oggenc: Set the right AVOption size for the pref_duration option

oggenc: Set the right AVOption size for the pref_duration option

On big endian machines, the default value set via the faulty
AVOption ended up as 2^32 times too big.

This fixes the fate-lavf-ogg test which currently is broken on
big endian machines, broken since 3831362. Since that commit,
a final zero-sized packet is written to the ogg muxer in that test,
which caused different flushing behaviour on little and big endian
depending on whether the pref_duration option was handled as it
should or not.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>