Fix clang build error

[jaschiu]

On my Gentoo Linux machine where I use Clang as the default C/C++ compiler, I get this build error for some reason:

/usr/include/glibmm-2.4/glibmm/ustring.h:1290:11: error: invalid operands to binary expression ('StreamType' (aka 'basic_ostringstream<char>') and 'const std::_Setfill<wchar_t>')
 1290 |   stream_ << value;
      |   ~~~~~~~ ^  ~~~~~
/usr/include/glibmm-2.4/glibmm/ustring.h:1405:7: note: in instantiation of function template specialization 'Glib::ustring::FormatStream::stream<std::_Setfill<wchar_t>>' requested here
 1405 |   buf.stream(a2);
      |       ^
../Waybar-0.12.0/src/modules/clock.cpp:361:32: note: in instantiation of function template specialization 'Glib::ustring::format<std::ios_base &(std::ios_base &), std::_Setfill<wchar_t>, std::_Setw, std::basic_string<char>>' requested here
  361 |           os << Glib::ustring::format((cldWPos_ != WS::LEFT || line == 0) ? std::left : std::right,
      |

The above change fixes it