Allow improper_ctypes_definitions in C API

This was enabled in rust-lang/rust#72700 but it looks like it's still
too noisy for it to be useful to us.

Author: alexcrichton

crates/c-api/src/lib.rs

@@ -2,6 +2,7 @@
 //! [Wasm C API](https://github.com/WebAssembly/wasm-c-api).
 
 #![allow(non_snake_case, non_camel_case_types, non_upper_case_globals)]
+#![allow(improper_ctypes_definitions)]
 
 // TODO complete the C API