Also mark the struct_field_attributes regression test

Rebase on top of PR 38814 made required this.

src/test/compile-fail/struct-field-attr-feature-gate.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// gate-test-struct_field_attributes
+
 struct Foo {
     present: (),
 }