This crate depends on native library libffi, but doesn't declare that dependency

So you get linking errors when using llvm-sys unless some other crate accidentally links to libffi.

This was found when testing some linking order changes in rustc in https://github.com/rust-lang/rust/pull/102832#issuecomment-1279772306.

I see there are some options for enabling it, but apparently they are not correctly enabled by default, because in a typical use scenario on x86_64-unknown-linux-gnu (see the link above and corresponding build failures in https://crater-reports.s3.amazonaws.com/pr-102832-1/index.html) llvm-sys does use symbols from libffi and you get errors if this dependency is not enabled.