Expose portable-atomic Feature` (#27)

Makes it easier to use `event-listener-strategy` on its own (e.g., `async-channel`) on platforms that require `portable-atomic`

Author: bushrat011899

Cargo.toml

@@ -20,6 +20,7 @@ pin-project-lite = "0.2.12"
 default = ["std"]
 loom = ["event-listener/loom"]
 std = ["event-listener/std"]
+portable-atomic = ["event-listener/portable-atomic"]
 
 [dev-dependencies]
 futures-lite = "2.0.0"