ENH: Add the ITK_INSTALL_LIBRARY_DIR to WrapITK.pth.

This allows the use of WrapITK in a virtualenv by symbolically linking to
WrapITK.pth in the site-packages directory.

Change-Id: Id27789be91fe5d48b7e3e3681fc56fb6b8c645ac
Merge topic 'wrapitk-pth-libraries'

7e645446 ENH: Add the ITK_INSTALL_LIBRARY_DIR to WrapITK.pth.