rust: workqueue: Enable execution of doctests

Having the Rust doctests enabled these workqueue tests are built but not
executed as the final callers of the print_*() functions are missing.
Add them.

The result is

        # rust_doctest_kernel_workqueue_rs_0.location: rust/kernel/workqueue.rs:35
    rust_doctests_kernel: The value is: 42
        ok 94 rust_doctest_kernel_workqueue_rs_0
        # rust_doctest_kernel_workqueue_rs_3.location: rust/kernel/workqueue.rs:78
    rust_doctests_kernel: The value is: 24
    rust_doctests_kernel: The second value is: 42
        ok 97 rust_doctest_kernel_workqueue_rs_3

Without this change the "The value ..." outputs are not there meaning
that this test code is not run.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/cb953202-0dbe-4127-8a8e-6a75258c2116@gmail.com
[ Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Dirk Behme

rust/kernel/workqueue.rs

@@ -69,6 +69,7 @@
 //! fn print_later(val: Arc<MyStruct>) {
 //!     let _ = workqueue::system().enqueue(val);
 //! }
+//! # print_later(MyStruct::new(42).unwrap());
 //! ```
 //!
 //! The following example shows how multiple `work_struct` fields can be used:
@@ -126,6 +127,8 @@
 //! fn print_2_later(val: Arc<MyStruct>) {
 //!     let _ = workqueue::system().enqueue::<Arc<MyStruct>, 2>(val);
 //! }
+//! # print_1_later(MyStruct::new(24, 25).unwrap());
+//! # print_2_later(MyStruct::new(41, 42).unwrap());
 //! ```
 //!
 //! C header: [`include/linux/workqueue.h`](srctree/include/linux/workqueue.h)