avcodec/jpeg2000: comment out unused variable

Fixes -Wunused-const-variable from
http://fate.ffmpeg.org/report.cgi?time=20150820031140&slot=arm64-darwin-clang-apple-5.1



Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>