Add simpleclock as fallback when hhdate is not available

ref https://github.com/Alexays/Waybar/issues/668

include/modules/simpleclock.hpp

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <fmt/format.h>
+#if FMT_VERSION < 60000
+#include <fmt/time.h>
+#else
+#include <fmt/chrono.h>
+#endif
+#include "ALabel.hpp"
+#include "util/sleeper_thread.hpp"
+
+namespace waybar::modules {
+
+class Clock : public ALabel {
+ public:
+  Clock(const std::string&, const Json::Value&);
+  ~Clock() = default;
+  auto update() -> void;
+
+ private:
+  util::SleeperThread thread_;
+};
+
+}  // namespace waybar::modules