rust: types: make doctests compilable/testable

Rust documentation tests are going to be build/run-tested
with the KUnit integration added in a future patch, thus
update them to make them compilable/testable so that we
may start enforcing it.

Reviewed-by: Martin Rodriguez Reboredo <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Reviewed-by: Björn Roy Baron <[e-mail]>
Reviewed-by: Alice Ryhl <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

rust/kernel/types.rs

···
 /// In the example below, we have multiple exit paths and we want to log regardless of which one is
 /// taken:
 /// ```
-/// # use kernel::ScopeGuard;
+/// # use kernel::types::ScopeGuard;
 /// fn example1(arg: bool) {
 ///     let _log = ScopeGuard::new(|| pr_info!("example1 completed\n"));
 ///
···
 /// In the example below, we want to log the same message on all early exits but a different one on
 /// the main exit path:
 /// ```
-/// # use kernel::ScopeGuard;
+/// # use kernel::types::ScopeGuard;
 /// fn example2(arg: bool) {
 ///     let log = ScopeGuard::new(|| pr_info!("example2 returned early\n"));
 ///
···
 /// In the example below, we need a mutable object (the vector) to be accessible within the log
 /// function, so we wrap it in the [`ScopeGuard`]:
 /// ```
-/// # use kernel::ScopeGuard;
+/// # use kernel::types::ScopeGuard;
 /// fn example3(arg: bool) -> Result {
 ///     let mut vec =
 ///         ScopeGuard::new_with_data(Vec::new(), |v| pr_info!("vec had {} elements\n", v.len()));