chore: fix linux LLVM paths

[cipherchabon]

On my machine I had to add a new path to locate the LLVM.
To locate the path I used the following command:

$ find /usr -iname "libclang.so" -print
/usr/lib/llvm-6.0/lib/libclang.so

I leave a link to the issue as a reference of the problem I had and the solution.

#14 (comment)

[thlorenz]

I assume this works more reliably than just passing the folders (even though I assume the issue actually lies with ffigen).
With all the tests passing in CI (for Ubuntu) it looks like nothing broke either, so just gonna merge this.

Thanks for the fix @cypherchabon

[thlorenz]

@cypherchabon just for future reference, please try to create the branch and push commits to your fork instead directly to this repo for now. Otherwise we might get confused with numerous branches by numerous developers here.
You can then create a PR from your fork to this repo.

[cipherchabon]

For a moment I thought about doing it through a fork, but then I remembered that you had asked us to please not fork. When in doubt I should have asked, but I'll do it next time.