avcodec/dnxhdenc: make header_prefix static const

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/dxtory: make def_lru[8] static

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpeg: make 2 outcommented tables static
avcodec/mdec: make block_index static const