rust: pin-init: feature-gate the stack_init_reuse test on the std feature

When trying to run `cargo check --all-targets --no-default-features`, an
error is reported by the test, as it cannot find the `std` crate. This
is to be expected, since the `--no-default-features` flag enables the
`no-std` behavior of the crate. Thus exclude the test in that scenario.

Link: 2813729cca
Link: https://lore.kernel.org/all/20250523125424.192843-4-lossin@kernel.org
[ Changed my author email address to @kernel.org. - Benno ]
Signed-off-by: Benno Lossin <lossin@kernel.org>

rust/pin-init/src/__internal.rs

@@ -188,6 +188,7 @@ impl<T> StackInit<T> {
 }
 
 #[test]
+#[cfg(feature = "std")]
 fn stack_init_reuse() {
     use ::std::{borrow::ToOwned, println, string::String};
     use core::pin::pin;