COMP: Fix redefined _POSIX_C_SOURCE warning

As detailed in kitware/VTK@0c8d479, the Python documentation
clearly states the proper approach:

"Since Python may define some pre-processor definitions which
affect the standard headers on some systems, you must include
Python.h before any standard headers are included."
(http://docs.python.org/c-api/intro.html#include-files)

Include "vtkPython.h" prior to any system header in all translation
units that need Python.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23950 3bd1e089-480b-0410-8dfb-8563597acbee