Fix native signature

Author: alexcrichton

crates/guest-rust/rt/src/async_support/waitable_set.rs

@@ -81,7 +81,7 @@ unsafe fn wait(_: u32, _: *mut [u32; 2]) -> u32 {
     unreachable!();
 }
 #[cfg(not(target_arch = "wasm32"))]
-unsafe fn poll(_: u32, _: *mut [u32; 3]) -> u32 {
+unsafe fn poll(_: u32, _: *mut [u32; 2]) -> u32 {
     unreachable!();
 }