rust: str: 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: Björn Roy Baron <bjorn3_gh@protonmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

rust/kernel/str.rs

@@ -213,6 +213,7 @@ impl fmt::Display for CStr {
     ///
     /// ```
     /// # use kernel::c_str;
+    /// # use kernel::fmt;
     /// # use kernel::str::CStr;
     /// # use kernel::str::CString;
     /// let penguin = c_str!("🐧");
@@ -241,6 +242,7 @@ impl fmt::Debug for CStr {
     ///
     /// ```
     /// # use kernel::c_str;
+    /// # use kernel::fmt;
     /// # use kernel::str::CStr;
     /// # use kernel::str::CString;
     /// let penguin = c_str!("🐧");
@@ -529,7 +531,7 @@ impl fmt::Write for Formatter {
 /// # Examples
 ///
 /// ```
-/// use kernel::str::CString;
+/// use kernel::{str::CString, fmt};
 ///
 /// let s = CString::try_from_fmt(fmt!("{}{}{}", "abc", 10, 20)).unwrap();
 /// assert_eq!(s.as_bytes_with_nul(), "abc1020\0".as_bytes());