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-android/src/lib.rs

@@ -71,6 +71,8 @@
 //!    event loop (as shown above).
 #![cfg(target_os = "android")]
 
+#![warn(clippy::exhaustive_enums)]
+
 mod event_loop;
 mod keycodes;