Rename ff_mpeg1_decode_block_intra to mpeg1_decode_block_intra and
make it static.  Add ff_mpeg1_decode_block_intra as plain extern
function calling mpeg1_decode_block_intra.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@18859 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b