Possible anachronism in CMake `target_include_directories`

[InonS]

When trying to compile, line 29 of the CMake file was giving me trouble. Changing the included item from ArrayFire::af to af solved the issue. I'm using CMake 3.10.2

See also:

1. https://cmake.org/cmake/help/latest/command/target_include_directories.html
2. https://gitlab.kitware.com/cmake/cmake/issues/15689