Fix VS 12.0 build error

This fix the following error:

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c:\python27\include\pymath.h(22) : warning C4273: 'round' : inconsistent dll linkage
        C:\Program Files\Microsoft Visual Studio 12.0\VC\INCLUDE\math.h(516) : see previous definition of 'round'
netifaces.c(406) : error C2275: 'PyObject' : illegal use of this type as an expression
        c:\python27\include\object.h(108) : see declaration of 'PyObject'
netifaces.c(406) : error C2065: 'py_family' : undeclared identifier
netifaces.c(407) : error C2065: 'py_family' : undeclared identifier