Fix non-binding let breaking build

[piotr-yuxuan]

$ cargo build --release
   Compiling keylightd v1.1.0 (/home/caocoa/Dropbox/Pierre/src/github.com/jonas-schievink/keylightd)
error: non-binding let on a synchronization lock
   --> src/main.rs:134:14
    |
134 |         let (_, result) = act
    |              ^ this lock is not assigned to a binding and is immediately dropped
    |
    = help: consider immediately dropping the value using `drop(..)` after the `let` statement
    = note: `#[deny(let_underscore_lock)]` on by default
help: consider binding to an unused variable to avoid immediately dropping the value
    |
134 |         let (_unused, result) = act
    |              ~~~~~~~

error: could not compile `keylightd` (bin "keylightd") due to 1 previous error

[tgrushka]

Yep, this fixes it for me.

[andypiper]

thanks, I am seeing this issue too.