unnecessary_cast for u8 as c_char on some platforms

On many platforms, c_char is an alias for i8. On some others, though, it's u8.

So when you have code like
```
const FOO: std::os::raw::c_char = b'f' as std::os::raw::c_char; 
```
which is necessary on those many platforms, clippy complains when run on one of those other platforms (e.g. aarch64 linux).