*poutbuf is non-const, so this casts it explicitly.
This suppresses -Wdiscarded-qualifiers seen in e.g
http://fate.ffmpeg.org/log.cgi?time=20150919100330&log=compile&slot=x86_64-archlinux-gcc-enableshared.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>