Make siphashc compile on Windows.

This is what I needed to change for siphashc to compile on my Windows machine.

Python: Python 2.7.3 (32-bit)
System: Windows 7 (64-bit)
Compiler: Microsoft Visual Studio 9.0

I also tried compiling it on Linux, without any problems.

Python: Python 2.7.6 (Anaconda 1.5.0) (64-bit)
System: Red Hat Enterprise Linux Server release 6.4 (Santiago), Linux 2.6.32, x86_64
Compiler: gcc 4.4.7 (Red Hat 4.4.7-4)