avcodec: add const qualifier to avcodec_find_best_pix_fmt2 args

avcodec_find_best_pix_fmt2 does not modify its first argument so may be
marked const.  This avoids a warning when passing in AVCodec.pix_fmts.

Signed-off-by: Dylan Simon <dylan@dylex.net>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>