pw: fix build on FreeBSD by using local eventfd definition

The Rust libc crate currently targets FreeBSD 10 (and will soon be updated
to target 12), but eventfd has only been added to FreeBSD 13.

Unfortunately the Rust ecosystem still has no solution for targeting
particular OS versions, so even after eventfd gets added to the libc crate,
that definition won't be usable until the targeted version changes again.

We'll have to use a local 'extern' eventfd definition.