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

從 CMakeLists.txt 第 29 行開始，使用 CMake 3.10.2 重現編譯問題。參考連結的 CMake 文件和 issue 15689，比較 ArrayFire::af 和 af 兩種形式。專案不需要所回報的 workaround 即可編譯通過，即表示完成。

由索引模型根據 Issue 內容生成。