Warn on exhaustive exported enums

Enable clippy::exhaustive_enums in every crate so new exported enums
are either #[non_exhaustive] or explicitly allowed as a closed set.

winit-x11/src/lib.rs

@@ -1,5 +1,7 @@
 //! # X11
 
+#![warn(clippy::exhaustive_enums)]
+
 use dpi::Size;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};