Possible anachronism in CMake `target_include_directories`

When trying to compile, [line 29 of the CMake file](https://github.com/arrayfire/arrayfire-ml/blob/master/CMakeLists.txt#L29) 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

Start at line 29 of CMakeLists.txt and reproduce the compile problem with CMake 3.10.2. Compare the ArrayFire::af and af forms using the linked CMake documentation and issue 15689. Done means the project compiles without requiring the reported workaround.

Written by the indexing model from the issue text.