gcc complains that the printf %x formatting instruction expects an
'unsigned int' but we pass a 'size_t'. We add the 'z' length formatting
specifier used for 'size_t'
Setting this is not required anymore as of commit 40cf1bbacc622 of
FFmpeg.
For users of older versions of the libavcodec library we guard the
change with an #if.
Silvan Jegen