Provide suggestion for `#![feature(default_field_values)]`

```
error[E0797]: base expression required after `..`
  --> $DIR/feature-gate-default-field-values.rs:62:21
   |
LL |     let x = Foo { .. };
   |                     ^
   |
help: add `#![feature(default_field_values)]` to the crate attributes to enable default values on `struct` fields
   |
LL + #![feature(default_field_values)]
   |
help: add a base expression here
   |
LL |     let x = Foo { ../* expr */ };
   |                     ++++++++++
```