Update kmac.c

fix nmake compiler error

```
crypto\kmac\kmac.c : warning treated as error - no object file generated
crypto\kmac\kmac.c : warning C4819: The file contains a character that cannot be represented in the current code page (936). Save the file in Unicode format to prevent data loss
```

CLA: trivial

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7846)