Ignore lint about rustfmt::skip

We support old compilers on which #[rustfmt::skip] is a parse error.
diff --git a/src/lib.rs b/src/lib.rs
index dbbeda2..8875da0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -246,6 +246,7 @@
         block_in_if_condition_stmt,
         const_static_lifetime,
         cyclomatic_complexity,
+        deprecated_cfg_attr,
         doc_markdown,
         eval_order_dependence,
         large_enum_variant,