Rollup merge of rust-lang#94220 - GuillaumeGomez:miniz-oxide-decl, r=Amanieu

Correctly handle miniz_oxide extern crate declaration

Fixes rust-lang#94219.

Follow-up of rust-lang#94122.

The `miniz_oxide` dependency is optional and therefore should allow be "imported" when it makes sense.

r? `@ivmarkov`

Author: matthiaskrgr

library/std/src/lib.rs

@@ -366,6 +366,7 @@ extern crate unwind;
 
 #[doc(masked)]
 #[allow(unused_extern_crates)]
+#[cfg(feature = "miniz_oxide")]
 extern crate miniz_oxide;
 
 // During testing, this crate is not actually the "real" std library, but rather