switched stat to stati64 to support files larger than 4 GB

Hi Krzysztof,

I tried out you Windows port of pigz and it works well. Thanks for the port. The only issue I ran into, was that I couldn't compress files larger than 4GB. The reason was that the standard stat calls only use a 32bit file length type. I switched the defines in wincompat.h to the versions supporting large files and created a pull request for the changes.

Best,
Andreas