Merge pull request #1427 from google/fix-extern-warning

Fix warning about extern with nightly.

Author: adetaylor

integration-tests/src/lib.rs

@@ -431,7 +431,7 @@ pub fn do_run_test_manual(
 
     rust_code.append_all(quote! {
         #[link(name="autocxx-demo")]
-        extern {}
+        extern "C" {}
     });
     info!("Unexpanded Rust: {}", rust_code);