Silence warnings on 'extern crate proc_macro'.

Author: SergioBenitez

contrib/codegen/src/lib.rs

@@ -26,6 +26,7 @@
 //! DATABASE_NAME := (string literal)
 //! </pre>
 
+#[allow(unused_extern_crates)]
 extern crate proc_macro;
 
 #[allow(unused_imports)]

core/codegen/src/lib.rs

@@ -59,7 +59,7 @@
 //! ```
 
 #[macro_use] extern crate quote;
-extern crate proc_macro;
+#[allow(unused_extern_crates)] extern crate proc_macro;
 
 use rocket_http as http;