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>