Squash improper_ctypes warnings on nightly

Lots more warnings are showing up on nightly compilers due to a recent
change. I've opened rust-lang/rust#66373 on the compiler side for this
as well.

crates/wasi/src/lib.rs

@@ -1,3 +1,5 @@
+#![allow(improper_ctypes)]
+
 extern crate alloc;
 
 mod instantiate;