only include the sa_restorer field when not on mac

the struct doesn't have this field on mac:
https://docs.rs/libc/latest/x86_64-apple-darwin/libc/struct.sigaction.html

fortunately, we just set it to None here anyway, even on linux

src/key_handlers.rs

@@ -534,6 +534,7 @@ pub(crate) unsafe extern "C" fn spawn(arg: *const Arg) {
  // MaybeUninit the whole sigaction if I can avoid it
  sa_mask: std::mem::zeroed(),
  sa_flags: 0,
+ #[cfg(not(target_os = "macos"))]
  sa_restorer: None,
  };
  libc::sigemptyset(&mut sa.sa_mask);

src/main.rs

@@ -232,6 +232,7 @@ fn setup() {
  sa_flags: libc::SA_NOCLDSTOP
  | libc::SA_NOCLDWAIT
  | libc::SA_RESTART,
+ #[cfg(not(target_os = "macos"))]
  sa_restorer: None,
  };
  libc::sigemptyset(&mut sa.sa_mask);