patch 9.1.0530: xxd: MSVC warning about non-ASCII character

patch 9.1.0530: xxd: MSVC warning about non-ASCII character

Problem:  xxd: MSVC warning about non-ASCII character
Solution: Specify source-charset:utf-8 in Makefile (Ken Takata)

xxd.c has non-ASCII-character comments. This causes the following
warning on MSVC:
```
warning C4819: The file contains a character that cannot be represented
in the current code page (932). Save the file in Unicode format to
prevent data loss.
```

Add the `/source-charset:utf-8` option to avoid this.

closes: #15119

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>