Fix toggle for header-clock

I'm not 100% sure why, but with this change, the listener which calls
`CheckHeaderClockEnabled` will be invoked *before* the listener which
persists the setting.

If the setting is persisted before that, the clock doesn't show up when
enabling it in the settings-menu and appears/disappears in the exact
opposite way the setting is true/false.

public/js/grocy_clock.js

@@ -1,4 +1,4 @@
-$("#show-clock-in-header").on("change", function()
+$(document).on("change", "#show-clock-in-header", function()
 {
 	CheckHeaderClockEnabled();
 });