AP_Landing: Directly include needed headers

libraries/AP_Landing/AP_Landing.cpp

@@ -19,6 +19,7 @@
 
 #include "AP_Landing.h"
 #include <GCS_MAVLink/GCS.h>
+#include <AP_AHRS/AP_AHRS.h>
 
 // table of user settable parameters
 const AP_Param::GroupInfo AP_Landing::var_info[] = {

libraries/AP_Landing/AP_Landing_Deepstall.cpp

@@ -22,6 +22,7 @@
 #include <AP_HAL/AP_HAL.h>
 #include <SRV_Channel/SRV_Channel.h>
 #include <AP_Common/Location.h>
+#include <AP_AHRS/AP_AHRS.h>
 
 // table of user settable parameters for deepstall
 const AP_Param::GroupInfo AP_Landing_Deepstall::var_info[] = {

libraries/AP_Landing/AP_Landing_Slope.cpp

@@ -21,6 +21,7 @@
 #include <GCS_MAVLink/GCS.h>
 #include <AP_HAL/AP_HAL.h>
 #include <AP_LandingGear/AP_LandingGear.h>
+#include <AP_AHRS/AP_AHRS.h>
 #include <AP_GPS/AP_GPS.h>
 
 void AP_Landing::type_slope_do_land(const AP_Mission::Mission_Command& cmd, const float relative_altitude)