Ignore intentional unconstructed struct in ui test helper crate

[dtolnay]

The point of this struct is testing what happens if #[ref_cast_custom] is not used from the same crate as the derive(RefCastCustom) struct.

ref-cast/tests/ui/cross-crate.rs

Lines 1 to 5 in daead7d

	use ref_cast::ref_cast_custom;
	use ref_cast_test_suite::Struct;
	#[ref_cast_custom]
	fn ref_cast(s: &str) -> &Struct;

ref-cast/tests/ui/cross-crate.stderr

Lines 1 to 5 in daead7d

	error[E0639]: cannot create non-exhaustive struct using struct expression
	--> tests/ui/cross-crate.rs:5:32
	|
	5 | fn ref_cast(s: &str) -> &Struct;
	| ^

Since rust-lang/rust#125572, it has started producing a new dead_code warning.

warning: struct `Struct` is never constructed
 --> tests/helper/lib.rs:5:12
  |
5 | pub struct Struct(#[allow(dead_code)] str);
  |            ^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default