curses: Fixes compiler error that complain don't have langinfo.h on msys2/mingw

msys2/mingw lacks the POSIX-required langinfo.h.

gcc test.c -DNCURSES_WIDECHAR -I/mingw64/include/ncursesw -pipe -lncursesw -lgnurx -ltre -lintl -liconv
test.c:4:10: fatal error: langinfo.h: No such file or directory
    4 | #include <langinfo.h>
      |          ^~~~~~~~~~~~
compilation terminated.

So we using g_get_codeset instead of nl_langinfo(CODESET)

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201012234348.1427-3-luoyonggang@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>