Ignore clippy::struct_field_names lint

```
error: all fields have the same prefix: `proj`
   --> pin-project-internal/src/pin_project/derive.rs:277:1
    |
277 | / struct ProjectedVariants {
278 | |     proj_variants: TokenStream,
279 | |     proj_ref_variants: TokenStream,
280 | |     proj_own_variants: TokenStream,
...   |
283 | |     proj_own_arms: TokenStream,
284 | | }
    | |_^
    |
    = help: remove the prefixes
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#struct_field_names
    = note: `-D clippy::struct-field-names` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::struct_field_names)]`

error: all fields have the same prefix: `proj`
   --> pin-project-internal/src/pin_project/derive.rs:287:1
    |
287 | / struct ProjectedFields {
288 | |     proj_pat: TokenStream,
289 | |     proj_body: TokenStream,
290 | |     proj_own_body: TokenStream,
...   |
293 | |     proj_own_fields: TokenStream,
294 | | }
    | |_^
    |
    = help: remove the prefixes
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#struct_field_names
```

pin-project-internal/src/lib.rs

@@ -15,6 +15,7 @@
     clippy::needless_doctest_main,
     clippy::similar_names,
     clippy::single_match_else,
+    clippy::struct_field_names,
     clippy::too_many_lines
 )]