Add use serde_derive::* to generated modules

This fixes an issue in Rust 2018 Edition where the serde macros are not 
in scope in the generated modules.

Author: aergonaut

graphql_client_codegen/src/codegen.rs

@@ -145,6 +145,8 @@ pub fn response_for_query(
     let response_derives = context.response_derives();
 
     Ok(quote! {
+        use serde_derive::*;
+
         #[allow(dead_code)]
         type Boolean = bool;
         #[allow(dead_code)]