allow 'extern crate alloc' even when that is the crates.io crate

Author: RalfJung

src/lib.rs

@@ -52,6 +52,7 @@ pub enum DefaultHashBuilder {}
 extern crate std;
 
 #[cfg_attr(test, macro_use)]
+#[cfg_attr(feature = "rustc-dep-of-std", allow(unused_extern_crates))]
 extern crate alloc;
 
 #[cfg(feature = "nightly")]