AP_Mission: tidy includes

libraries/AP_Mission/AP_Mission.h

@@ -12,11 +12,12 @@
  */
 #pragma once
 
+#include <AP_HAL/AP_HAL.h>
+
 #ifndef HAL_MISSION_ENABLED
 #define HAL_MISSION_ENABLED 1
 #endif
 
-#include <AP_HAL/AP_HAL.h>
 #include <GCS_MAVLink/GCS_MAVLink.h>
 #include <AP_Math/AP_Math.h>
 #include <AP_Common/AP_Common.h>

libraries/AP_Mission/AP_Mission_ChangeDetector.cpp

@@ -3,7 +3,7 @@
 
 #include "AP_Mission_ChangeDetector.h"
 
-extern const AP_HAL::HAL& hal;
+#if HAL_MISSION_ENABLED
 
 // detect external changes to mission
 bool AP_Mission_ChangeDetector::check_for_mission_change()
@@ -59,3 +59,4 @@ bool AP_Mission_ChangeDetector::check_for_mission_change()
     return cmds_changed && !curr_cmd_idx_changed;
 }
 
+#endif  // AP_MISSION_ENABLED