AntennaTracker: include SITL only for SITL board

Include board-specific files only when the board is used. Since these
should be exceptional cases, let the includer handle the ifdef instead
of putting ifdefs in every platform-specific header.

In the future we should evaluate whether the HAL for the board should
instantiate this.

AntennaTracker/Tracker.h

@@ -51,7 +51,6 @@
 #include <AP_SerialManager/AP_SerialManager.h>   // Serial manager library
 #include <AP_Declination/AP_Declination.h> // ArduPilot Mega Declination Helper Library
 #include <DataFlash/DataFlash.h>
-#include <SITL/SITL.h>
 #include <PID/PID.h>
 #include <AP_Scheduler/AP_Scheduler.h>       // main loop scheduler
 #include <AP_NavEKF/AP_NavEKF.h>
@@ -76,6 +75,10 @@
 #include "Parameters.h"
 #include <GCS_MAVLink/GCS.h>
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+#include <SITL/SITL.h>
+#endif
+
 class Tracker : public AP_HAL::HAL::Callbacks {
 public:
     friend class GCS_MAVLINK;