Use correct calling convention on Windows.

[estyrke]

OpenCL.dll uses stdcall calling convention (source), which means we need ctypes.windll instead of ctypes.cdll on Windows.

Verified that this fix works on Windows 10 with nVidia OpenCL library. Without this patch, I get

    clGetPlatformIDs.call(0, None, byref(num_platforms))
ValueError: Procedure called with not enough arguments (12 bytes missing) or wrong calling convention