bugfix: Fix check_cfg warning on nightly

Signed-off-by: John Nunley <dev@notgull.net>

Cargo.toml

@@ -14,6 +14,9 @@ keywords = ["mio", "epoll", "kqueue", "iocp"]
 categories = ["asynchronous", "network-programming", "os"]
 exclude = ["/.*"]
 
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(polling_test_poll_backend)'] }
+
 [[bench]]
 name = "io"
 harness = false

src/os/unix.rs

@@ -1,6 +1,5 @@
 //! Functionality that is only available for `unix` platforms.
 
-#[cfg(not(async_io_no_io_safety))]
 use std::os::unix::io::BorrowedFd;
 
 /// Get a file descriptor that can be used to wait for readiness in an external runtime.
@@ -41,7 +40,6 @@ use std::os::unix::io::BorrowedFd;
 /// #     pub fn register(_: BorrowedFd<'_>) {}
 /// # }
 /// ```
-#[cfg(not(async_io_no_io_safety))]
 pub fn reactor_fd() -> Option<BorrowedFd<'static>> {
     cfg_if::cfg_if! {
         if #[cfg(all(