Fix VP8 decoder dependencies

Fix VP8 decoder dependencies

This reverts rev 24674 - the VP8 decoder actually depends on cabac.o.
vp8.c includes vp56.h, which includes cabac.h, which has inline functions
that reference tables from cabac.c.

This fixes compilation with --disable-everything --enable-decoder=vp8.

Originally committed as revision 24692 to svn://svn.ffmpeg.org/ffmpeg/trunk