Add core::cfg to the prelude

This is not always present when we are building as `rustc-dep-of-std`.

(backport <#4782>)
(cherry picked from commit 919d0d4)

Author: tgross35

src/macros.rs

@@ -87,6 +87,7 @@ macro_rules! prelude {
             pub(crate) use core::prelude::v1::derive;
             #[allow(unused_imports)]
             pub(crate) use core::{
+                cfg,
                 fmt,
                 hash,
                 iter,