Fix clippy::needless_borrow warning

```
error: the borrowed expression implements the required traits
   --> src/lib.rs:212:57
    |
212 |                     signal_hook::iterator::Signals::new(&[signal_hook::consts::SIGCHLD])
    |                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `[signal_hook::consts::SIGCHLD]`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
```

Author: taiki-e

src/lib.rs

@@ -209,7 +209,7 @@ impl Child {
 
                 // Make sure the signal handler is registered before interacting with the process.
                 SIGNALS.get_or_init_blocking(|| Mutex::new(
-                    signal_hook::iterator::Signals::new(&[signal_hook::consts::SIGCHLD])
+                    signal_hook::iterator::Signals::new([signal_hook::consts::SIGCHLD])
                         .expect("cannot set signal handler for SIGCHLD"),
                 ));