Codechange: Declare all IntervalTimers const, which can be const.

src/airport_gui.cpp

@@ -575,7 +575,7 @@ public:
 		CheckRedrawStationCoverage(this);
 	}
 
-	IntervalTimer<TimerGameCalendar> yearly_interval = {{TimerGameCalendar::YEAR, TimerGameCalendar::Priority::NONE}, [this](auto) {
+	const IntervalTimer<TimerGameCalendar> yearly_interval = {{TimerGameCalendar::YEAR, TimerGameCalendar::Priority::NONE}, [this](auto) {
 		this->InvalidateData();
 	}};
 };