Add expected configs for unexpected_cfgs (#1787)

Adds the two expected cfg flags for Mio.

Author: Thomasdezeeuw

Cargo.toml

@@ -107,3 +107,6 @@ required-features = ["os-poll", "net"]
 [[example]]
 name = "udp_server"
 required-features = ["os-poll", "net"]
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(mio_unsupported_force_poll_poll)', 'cfg(mio_unsupported_force_waker_pipe)'] }