[navoutformat.cpp] CodecID -> AVCodecID

This allows me to compile navcodec on Debian with libavformat 2.1.1 (from the deb-multimedia.org set of packages).

It's been called AVCodec for a while (libav/libav@2ff67c909c95903240c30405b0b231ba68f5407a, libav/libav@36ef5369ee9b336febc2c270f8718cec4476cb85). There was a backwards compatible #define, but that got removed in libav commit libav/libav@bdd1567c355a8092e7746ef99e831d579e34fa6a.

-David