Update mini-macro post proc macro stabilization

rust-lang/rust#52081 stabilized proc macros, but
quote is still unstable, so you need to explicitly enable that feature.

mini-macro/src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(proc_macro, proc_macro_non_items)]
+#![feature(use_extern_macros, proc_macro_quote, proc_macro_non_items)]
 extern crate proc_macro;
 
 use proc_macro::{TokenStream, quote};