Bug 1507833 - fixed an unused #[macro_use] warning in our rust code; r=nika

Differential Revision: https://phabricator.services.mozilla.com/D12159

--HG--
extra : moz-landing-system : lando

xpcom/rust/xpcom/xpcom_macros/src/lib.rs

@@ -131,7 +131,6 @@
 #[macro_use]
 extern crate quote;
 
-#[macro_use]
 extern crate syn;
 
 extern crate proc_macro;