fix(c-api) Fix how wasm_globaltype_t is implemented.

In `wasm.h`, `wasm_globaltype_t` is implemented with
`WASM_DECLARE_TYPE`, so with `WASM_DECLARE_VEC(globaltype, *)`. This
`*` means the C struct for the vector is defined:

```c
struct wasm_globaltype_vec_t {
    size_t size;
    wasm_globaltype_t** data;
}
```

The way we implement `wasm_globaltype_vec_t` in Rust is with the
`wasm_declare_vec!` macro. And it is wrong. We must use
`wasm_declared_boxed_vec!`.

lib/c-api/src/wasm_c_api/types/global.rs

@@ -46,7 +46,7 @@ impl wasm_globaltype_t {
     }
 }
 
-wasm_declare_vec!(globaltype);
+wasm_declare_boxed_vec!(globaltype);
 
 #[no_mangle]
 pub unsafe extern "C" fn wasm_globaltype_new(