Fix clippy::derivable_impls lint

[pekkarr]

CI for #3313 failed because of this clippy lint.

warning: this `impl` can be derived
    --> bindgen/codegen/mod.rs:4132:1
     |
4132 | / impl Default for NonCopyUnionStyle {
4133 | |     fn default() -> Self {
4134 | |         Self::BindgenWrapper
4135 | |     }
4136 | | }
     | |_^
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#derivable_impls
     = note: `#[warn(clippy::derivable_impls)]` on by default
help: replace the manual implementation with a derive attribute and mark the default variant
     |
4111 + #[derive(Default)]
4112 ~ pub enum NonCopyUnionStyle {
4113 |     /// Wrap members in a type generated by `bindgen`.
4114 ~     #[default]
4115 ~     BindgenWrapper,
     |

warning: this `impl` can be derived
   --> bindgen/lib.rs:182:1
    |
182 | / impl Default for Formatter {
183 | |     fn default() -> Self {
184 | |         Self::Rustfmt
185 | |     }
186 | | }
    | |_^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#derivable_impls
help: replace the manual implementation with a derive attribute and mark the default variant
    |
172 + #[derive(Default)]
173 ~ pub enum Formatter {
174 |     /// Do not format the bindings.
175 |     None,
176 |     /// Use `rustfmt` to format the bindings.
177 ~     #[default]
178 ~     Rustfmt,
    |

[pekkarr]

r? @emilio

[emilio]

Sorry for the dupe review, github is not great with crappy internet and I'm traveling this weekend.