chore: silence clippy

It appears that clippy's dead code detection has gotten better. This
commit fixes winit's code to match.

Signed-off-by: John Nunley <dev@notgull.net>

src/platform_impl/linux/x11/dnd.rs

@@ -23,10 +23,10 @@ pub enum DndState {
 #[derive(Debug)]
 pub enum DndDataParseError {
     EmptyData,
-    InvalidUtf8(Utf8Error),
-    HostnameSpecified(String),
-    UnexpectedProtocol(String),
-    UnresolvablePath(io::Error),
+    InvalidUtf8(#[allow(dead_code)] Utf8Error),
+    HostnameSpecified(#[allow(dead_code)] String),
+    UnexpectedProtocol(#[allow(dead_code)] String),
+    UnresolvablePath(#[allow(dead_code)] io::Error),
 }
 
 impl From<Utf8Error> for DndDataParseError {