Also include win/* when creating sdist package

[nils-werner]

I have noticed the following under windows

• pip install pysoundfile does not work because the dll files do not exist and shutil.copy2 fails.
• Cloning the repository from GitHub and doing pip install -e . does work, because this way, the dll files do exist.

This has to do with the win/ directory not being included in MANIFEST.in and thusly not being included in the tar.gz file when you do python setup.py sdist.

This PR fixes that, making sure that the dll files are included in future PyPI releases of this package.

[mgeier]

For reference: #88, #98.

[nils-werner]

Another solution could be to catch errors in that move section of setup.py. Because currently, no matter if I have libsndfile anywhere or not, pip install pysoundfile just fails.