ArduPlane: 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.

ArduPlane/Plane.h

@@ -69,7 +69,6 @@
 #include <AP_Mount/AP_Mount.h>           // Camera/Antenna mount
 #include <AP_Declination/AP_Declination.h> // ArduPilot Mega Declination Helper Library
 #include <DataFlash/DataFlash.h>
-#include <SITL/SITL.h>
 #include <AP_Scheduler/AP_Scheduler.h>       // main loop scheduler
 
 #include <AP_Navigation/AP_Navigation.h>
@@ -104,6 +103,10 @@
 
 #include "Parameters.h"
 
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+#include <SITL/SITL.h>
+#endif
+
 /*
   a plane specific arming class
  */