Allow only unused_extern_crates instead of the whole rust_2018_idioms lint group in serde_derive-generated code

test_suite/tests/expand/ser_enum.expanded.rs

@@ -13,7 +13,7 @@ where
 #[doc(hidden)]
 #[allow(non_upper_case_globals, unused_attributes, unused_qualifications)]
 const _: () = {
-    #[allow(rust_2018_idioms, clippy::useless_attribute)]
+    #[allow(unused_extern_crates, clippy::useless_attribute)]
     extern crate serde as _serde;
     #[automatically_derived]
     impl<'a, B: 'a, C: 'a, D> _serde::Serialize for SerEnum<'a, B, C, D>