Accept WAV files in K3bLibsndfileDecoderFactory::canDecode

The comment said
   we exclude only WAVE as there is another plugin for this
i guess refering to the K3bWaveDecoder, but with the file from bug
399056 K3bWaveDecoder doesn't know how to handler it and
K3bLibsndfileDecoder does

Also K3bWaveDecoder is always running first than K3bLibsndfileDecoder in
AudioDecoderFactory::createDecoder so the new code is only really used
if the other one fails

BUGS: 399056