Include wchar.h before wrapping memchr

On mingw, wmemcpy() is defined as an inline function using
memcpy(), and the static inline wrapper causes a warning.

```
In file included from include/ruby/ruby.h:39,
                 from win32/file.c:5:
include/ruby/internal/memory.h:284:16: warning: 'ruby_nonempty_memcpy' is static but used in inline function 'wmemcpy' which is not static
  284 | #define memcpy ruby_nonempty_memcpy
      |                ^~~~~~~~~~~~~~~~~~~~
```

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