Suppress match_wildcard_for_single_variants clippy false positive

https://github.com/rust-lang/rust-clippy/issues/6984

    error: wildcard matches only a single variant and will also match any future added variants
        --> serde_derive/src/internals/attr.rs:1918:9
         |
    1918 |         _ => {}
         |         ^ help: try this: `syn::Type::__TestExhaustive(_)`
         |
    note: the lint level is defined here
        --> serde_derive/src/lib.rs:18:22
         |
    18   | #![deny(clippy::all, clippy::pedantic)]
         |                      ^^^^^^^^^^^^^^^^
         = note: `#[deny(clippy::match_wildcard_for_single_variants)]` implied by `#[deny(clippy::pedantic)]`
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_wildcard_for_single_variants

    error: wildcard matches only a single variant and will also match any future added variants
       --> serde_derive/src/internals/receiver.rs:153:13
        |
    153 |             _ => {}
        |             ^ help: try this: `Type::__TestExhaustive(_)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_wildcard_for_single_variants

    error: wildcard matches only a single variant and will also match any future added variants
       --> serde_derive/src/bound.rs:190:17
        |
    190 |                 _ => {}
        |                 ^ help: try this: `syn::Type::__TestExhaustive(_)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_wildcard_for_single_variants

serde_derive/src/lib.rs

@@ -43,6 +43,8 @@
     clippy::let_underscore_drop,
     clippy::map_err_ignore,
     clippy::match_same_arms,
+    // clippy bug: https://github.com/rust-lang/rust-clippy/issues/6984
+    clippy::match_wildcard_for_single_variants,
     clippy::module_name_repetitions,
     clippy::must_use_candidate,
     clippy::option_if_let_else,