Add #include for offsetof() macro in generated Python wrappers

The generated Python wrappers use the C offsetof() macro, but don't #include the
header that defines the macro (<cstddef>). For example, see
https://gitlab.kitware.com/vtk/vtk/blob/636aa17/Wrapping/Tools/vtkWrapPythonClass.c#L553.

This commit adds the proper header to the generated Python wrappers. Without
this, wrapping could fail unless <cstddef> is indirectly included through the
header of the class being wrapped. An example error message is:

    Libs/MRML/Core/vtkMRMLSubjectHierarchyConstantsPython.cxx:853:40: error: ‘offsetof’ was not declared in this scope
       offsetof(PyVTKObject, vtk_weakreflist), // tp_weaklistoffset

Author: msmolens

Wrapping/Tools/vtkWrapPython.c

@@ -385,6 +385,7 @@ int main(int argc, char *argv[])
           "#include \"vtkPythonArgs.h\"\n"
           "#include \"vtkPythonOverload.h\"\n"
           "#include \"vtkConfigure.h\"\n"
+          "#include <cstddef>\n"
           "#include <sstream>\n");
 
   /* vtkPythonCommand is needed to wrap vtkObject.h */