COMP: Make sure ITKFactoryRegistration_INCLUDE_DIRS is exposed to extension.

The expected include directories are already being included through the
help of the variable "Slicer_Libs_INCLUDE_DIRS"

That said, appending it to ITK_INCLUDE_DIRS will ensure that module
linking only against ITK will have all the elements allowing to
register ITK IOFactories available to them.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21753 3bd1e089-480b-0410-8dfb-8563597acbee