Fix build warnings

[zjeffer]

Fixes this warning:

In file included from ../waybar-git/src/modules/systemd_failed_units.cpp:1:
../waybar-git/include/modules/systemd_failed_units.hpp: In constructor ‘waybar::modules::SystemdFailedUnits::SystemdFailedUnits(const std::string&, const Json::Value&)’:
../waybar-git/include/modules/systemd_failed_units.hpp:23:30: warning: ‘waybar::modules::SystemdFailedUnits::nr_failed_user’ will be initialized after [-Wreorder]
   23 |   uint32_t nr_failed_system, nr_failed_user;
      |                              ^~~~~~~~~~~~~~
../waybar-git/include/modules/systemd_failed_units.hpp:22:15: warning:   ‘std::string waybar::modules::SystemdFailedUnits::last_status’ [-Wreorder]
   22 |   std::string last_status;
      |               ^~~~~~~~~~~
../waybar-git/src/modules/systemd_failed_units.cpp:13:1: warning:   when initialized here [-Wreorder]
   13 | SystemdFailedUnits::SystemdFailedUnits(const std::string& id, const Json::Value& config)
      | ^~~~~~~~~~~~~~~~~~