Adding include/vtk-8.1 directory as INTERFACE_INCLUDE_DIRECTORIES

This patch corrects using VTK as imported target in dependent projects. With this patch, VTKTargets.cmake includes ${VTK_INSTALL_INCLUDE_DIR} as INTERFACE_INCLUDE_DIRECTORY, which was missing. Therefore, a target "A" indirectly depending on VTK (e.g. via an intermediate VTK-based library "B") failed since VTK headers were not found. A technical "hack" would be to find VTK in "A" and include VTK_USE_FILE, but this would be conceptionally wrong. This patch provides a solution and helps make the inclusion of VTK_USE_FILE a little more obsolete.