x11-wm/blackbox: fix build with libc++ 19

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes x11-wm/blackbox to fail to compile with clang 19 and libc++
19, resulting in errors similar to:

  /usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined
template 'std::char_traits<unsigned int>'
    820 |   static_assert(is_same<_CharT, typename
traits_type::char_type>::value,
        |                                          ^
  ./Menu.hh:82:13: note: in instantiation of template class
'std::basic_string<unsigned int>' requested here
     82 |     ustring lbl;
        |             ^