submodules: update clippy from f5d868c to 2f6881c

````
missed another one in the README
run "util/dev update_lints"
rust-lang-nursery/rust-clippy => rust-lang/rust-clippy
Address 'clippy::single-match' dogfood lint
Fix nit
Address travis CI lint failure
Update trivially_copy_pass_by_ref with Trait stderr output
issue#3318 run trivially_copy_pass_by_ref for traits
Update trivially_copy_pass_by_ref with Trait examples
Fix awkward wording
Document how to lint local Clippy changes with locally built Clippy
Enable rustup clippy to refer to the correct documentation
rustup #52591
remove unused allow() attributes, NFC
Add regression test
Don't emit suggestion when inside of a macro
````

Author: matthiaskrgr