Temporarily ignore collection_is_never_read on FlattenSkipDeserializing

[dtolnay]

@Mingun, this lint fires since #2567. Could you take a look at what should be generated for such a struct?

serde/test_suite/tests/test_gen.rs

Lines 723 to 727 in fc55ac7

	#[derive(Serialize, Deserialize)]
	pub struct FlattenSkipDeserializing<T> {
	#[serde(flatten, skip_deserializing)]
	flat: T,
	}

error: collection is never read
   --> test_suite/tests/test_gen.rs:723:25
    |
723 |     #[derive(Serialize, Deserialize)]
    |                         ^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collection_is_never_read
note: the lint level is defined here
   --> test_suite/tests/test_gen.rs:23:9
    |
23  | #![deny(clippy::collection_is_never_read)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

[Mingun]

Hm, that lints does not checked on CI? Why didn't it fired when CI ran #2567?

I'm finishing a PR to fix that and will open it soon.