COMP: Fix build error fixing regression in SimpleITK installation

This commit fixes a regression introduced in previous commit. It turns
that SimpleITK install has been updated in SimpleITK/SimpleITK@2fc59f9
(Moving Python Wrapping to sub-directory).

This commit fixes the following error:

-- SimpleITK: SimpleITK_WORKING_DIR: /path/to/SimpleITK-build/SimpleITK-build/Wrapping
-- SimpleITK: /path/to/python-install/bin/SlicerPython;PythonPackage/setup.py;install
-- SimpleITK: Errors detected - See below.

/path/to/python-install/bin/python: can't open file 'PythonPackage/setup.py': [Errno 2] No such file or directory

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24912 3bd1e089-480b-0410-8dfb-8563597acbee