xbmc/utils/BitstreamConverter.{cpp,h}: s/CodecID/AVCodecID/, fixes compile with external ffmpeg >=2.0

Just found out about a compile error with external ffmpeg (>=2.0) on gentoo:

    CPP     xbmc/utils/BitstreamConverter.o
    In file included from BitstreamConverter.cpp:25:0:
    BitstreamConverter.h:136:31: error: use of enum 'CodecID' without previous declaration
    BitstreamConverter.cpp:299:37: error: use of enum 'CodecID' without previous declaration
    BitstreamConverter.cpp: In member function 'bool CBitstreamConverter::Open(int, uint8_t*, int, bool)':
    BitstreamConverter.cpp:303:13: error: invalid conversion from 'int' to 'AVCodecID' [-fpermissive]
    make[1]: *** [BitstreamConverter.o] Error 1
    make: *** [xbmc/utils/utils.a] Error 2

This small patch fixes compiling and brings "CodecID" on par with everything else to "AVCodecID". Build-tested with external ffmpeg on gentoo/amd64 and on Ubuntu Precise/amd64 with internal ffmpeg. Probably should be build-tested on iOS/OSX/Android/Win32.