locale.c: Win32: We don't know the locale here

And, so we don't know whether it is a UTF-8 locale or not.  So use the
proper enum value to indicate that.

locale.c

@@ -5210,7 +5210,7 @@ Perl_get_win32_message_utf8ness(pTHX_ const char * string)
 
 #    ifdef USE_LOCALE_CTYPE
 
-    return get_locale_string_utf8ness_i(string, LOCALE_IS_UTF8,
+    return get_locale_string_utf8ness_i(string, LOCALE_UTF8NESS_UNKNOWN,
                                         NULL, LC_CTYPE_INDEX_);
 #    else