[rust] Avoid warning for unused fields

Upstream rustc has expanded the unused field lint to to not count
usages performed by derived Clone and Debug implementations.
https://github.com/rust-lang/rust/pull/85200

This CL marks such fields with an #[allow(unused)] tag.

Bug: 84550
Change-Id: I26906946321be77e912c49aee48d938b80d1c204
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/rust-mirrors/rustyline/+/581061
Reviewed-by: Tyler Mandry <tmandry@google.com>
diff --git a/src/tty/fuchsia.rs b/src/tty/fuchsia.rs
index a6be6b1..796f8f6 100644
--- a/src/tty/fuchsia.rs
+++ b/src/tty/fuchsia.rs
@@ -61,8 +61,10 @@
 
 #[derive(Clone, Debug)]
 pub struct FuchsiaTerminal {
+    #[allow(unused)]
     unsupported: bool,
     stdin_isatty: bool,
+    #[allow(unused)]
     stdstream_isatty: bool,
     pub(crate) color_mode: ColorMode,
     stream_type: OutputStreamType,