Mark platform extension enums as #[non_exhaustive]

OS-defined sets and error enums in the platform crates can now gain
variants without a breaking change.

winit-x11/src/dnd.rs

@@ -22,6 +22,7 @@ pub enum DndState {
 }
 
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum UriListParseError {
     EmptyData,
     InvalidUtf8(#[allow(dead_code)] Utf8Error),

winit-x11/src/lib.rs

@@ -32,6 +32,7 @@ pub use dnd::{Selection, SelectionReader, SelectionType, UriListParseError};
 /// [`_NET_WM_WINDOW_TYPE`](https://specifications.freedesktop.org/wm-spec/wm-spec-1.5.html).
 #[derive(Debug, Default, Copy, Clone, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[non_exhaustive]
 pub enum WindowType {
     /// A desktop feature. This can include a single window containing desktop icons with the same
     /// dimensions as the screen, allowing the desktop environment to have full control of the