fix ament_export_libraries

[borongyuan]

I would recommend using ament_cmake_auto to write CMakeLists files. This will greatly simplify CMakeLists files, normalize file directories, and avoid such typo. The downside is that there seems to be a lack of documentation right now, but it doesn't really require much documentation. Here is a gist for reference: https://gist.github.com/dirk-thomas/a76f952d05e7b21b0128

[hellantos]

Thanks for the PR. I will check if and how we can make use of ament_cmake_auto.