avcodec/nvdec: More effort to make vp8 compile with gcc < 4.6

I'm told my prefix work-around wasn't enough to make it compile,
although I'm not sure why; I did some basic testing and that
approach appeared to work, but I'm not in a position to do a
full compile on CentOS 6 so I can't be sure of anything.

I have had it confirmed that the additional change to not use
named initialisers is enough to make it compile, so let's
throw that into the mix too.