Allow unused_extern_crates for now

Author: JohnTitor

mini-macro/src/lib.rs

@@ -1,5 +1,7 @@
 #![feature(proc_macro_quote, proc_macro_hygiene)]
 #![deny(rust_2018_idioms)]
+// FIXME: Remove this attribute once the weird failure is gone.
+#![allow(unused_extern_crates)]
 extern crate proc_macro;
 
 use proc_macro::{quote, TokenStream};