windows: Install headers in expected sub folder.

As specified in the "get_python_inc" function provided by the sysconfig
module, headers are expected to be inside the "include" directory.
See http://hg.python.org/cpython/file/6fc9103d55f0/Lib/distutils/sysconfig.py#l95

Note that for now only the (1) in-source build tree and (2) install tree
are compliant.