rust: pin-init: examples, tests: add conditional compilation in order to compile...

rust: pin-init: examples, tests: add conditional compilation in order to compile under any feature combination

In the CI, all examples & tests should be run under all feature
combinations. Currently several examples & tests use `std` without
conditionally enabling it. Thus make them all compile under any feature
combination by conditionally disabling the code that uses e.g. `std`.

Link: https://github.com/Rust-for-Linux/pin-init/pull/50/commits/fdfb70efddbc711b4543c850ee38a2f5a8d17cb6
Link: https://lore.kernel.org/all/20250523125424.192843-2-lossin@kernel.org
Signed-off-by: Benno Lossin <lossin@kernel.org>