Fix warning with nightly rust:

```
warning: cannot find macro `self_test` in this scope
  --> lib.rs:44:10
   |
44 | #![doc = self_test!(/**
   |          ^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #124535 <rust-lang/rust#124535>
   = help: import `macro_rules` with `use` to make it callable above its definition
   = note: `#[warn(out_of_scope_macro_calls)]` on by default

```

lib.rs

@@ -511,6 +511,8 @@ macro_rules! self_test {
     };
 }
 
+use self_test;
+
 // The following struct is inserted only during generation of the documentation in order to exploit doc-tests.
 // These doc-tests are used to check that invalid arguments to the `document_features!` macro cause a compile time error.
 // For a more principled way of testing compilation error, maybe investigate <https://docs.rs/trybuild>.