dead code warning misfires on unsafe pointers

[metajack]

This code in servo triggers the dead code warning. It claims struct NSAlert; is not used, even though it's used as part of a public struct a few lines later.

https://github.com/mozilla-servo/rust-alert/blob/master/macos.rs#L20-L28

[adrientetar]

cc @ktt3ja