Silence warning about mapping to unit type

This was flagged by `cargo clippy`:

    warning: called `map(f)` on an `Option` value where `f` is a closure
             that returns the unit type `()`

I decided to disable this because it was suggesting changes like the
following and I prefer the original:

    @@ -1 +1 @@
    -self.waker.take().map(|w| w.wake());
    +if let Some(w) = self.waker.take() { w.wake() }

src/lib.rs

@@ -93,6 +93,7 @@ compile_error!("at least one socket needs to be enabled"); */
 #![allow(clippy::match_like_matches_macro)]
 #![allow(clippy::redundant_field_names)]
 #![allow(clippy::identity_op)]
+#![allow(clippy::option_map_unit_fn)]
 
 #[cfg(feature = "alloc")]
 extern crate alloc;