size_t vs usize


### Input C/C++ Header

```C++
#include <stddef.h>
static size_t A = 5;
```

bindgen converts `size_t` to `usize. but I'm not sure that's correct. the unsafe guidelines(https://rust-lang.github.io/unsafe-code-guidelines/layout/scalars.html) say that usize==uintptr_t. but the C standard doesn't define that uintptr_t==size_t therefore usize != size_t.

Am I missing something?