Add missing include call

[myd7349]

CMAKE_INSTALL_INCLUDEDIR and CMAKE_INSTALL_LIBDIR used in CMakeLists.txt are defined in GNUInstallDirs.cmake. Without including this module, this command will fail:

$ cmake --build . --target install

-- Install configuration: "Debug"
-- Up-to-date: /cmake/Immer/ImmerConfig.cmake
CMake Error at cmake_install.cmake:60 (file):
file INSTALL given no DESTINATION

[arximboldi]

Thanks @myd7349 !