dd-trace-ot should not bundle unshaded com.kenai.jffi

`com.kenai.jffi` used to be a dependency in dd-trace-ot 0.x, then bundled as a shaded library in 1.x until 1.8.x, then unshaded from 1.9.x onward by this commit: https://github.com/DataDog/dd-trace-java/pull/4779

I understood the reason for not relocating it, but why not declare `com.kenai.jffi` as an external dependency instead? 