Add #![feature(staged_api)] attribute to E0539 error examples

Author: unexge

src/librustc_error_codes/error_codes/E0539.md

@@ -3,6 +3,8 @@ An invalid meta-item was used inside an attribute.
 Erroneous code example:
 
 ```compile_fail,E0539
+#![feature(staged_api)]
+
 #[rustc_deprecated(reason)] // error!
 #[unstable(feature = "deprecated_fn", issue = "123")]
 fn deprecated() {}
@@ -24,6 +26,8 @@ Meta items are the key-value pairs inside of an attribute.
 To fix these issues you need to give required key-value pairs.
 
 ```
+#![feature(staged_api)]
+
 #[rustc_deprecated(since = "1.39.0", reason = "reason")] // ok!
 #[unstable(feature = "deprecated_fn", issue = "123")]
 fn deprecated() {}