Suppress warning C4819: The file contains a character that cannot be represented in the current code page (932) on Japanese Windows

[kubo]

A comment in src/prefix.c contains two '\x97' bytes. They are em dash characters
in single-byte Windows code pages such as 1252. However they are invalid in
Japanese code page.

This PR replaces em dash with hyphen to suppress the warning C4819.

Without this, compilation using the win32 solution here fails with the following error:

1>prefix.c
1>C:\build\distorm\src\prefix.c: error C2220: 警告をエラーとして扱いました。'object' ファイルは生成されません。
1>C:\build\distorm\src\prefix.c: warning C4819: ファイルは、現在のコード ページ (932) で表示できない文字を含んでいます。データの損失を防ぐために、ファイルを Unicode 形式で保存してください

(translated to English)

1>prefix.c
1>C:\build\distorm\src\prefix.c: error C2220: warning treated as error - no object file generated
1>C:\build\distorm\src\prefix.c: 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.