AP_Landing: Fix AP_GPS.h include, use a ref to the singleton

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_GPS/AP_GPS.h>
 
 void AP_Landing::type_slope_do_land(const AP_Mission::Mission_Command& cmd, const float relative_altitude)
 {
@@ -92,14 +93,15 @@ bool AP_Landing::type_slope_verify_land(const Location &prev_WP_loc, Location &n
         (!rangefinder_state_in_range && wp_proportion >= 1) ||
         probably_crashed) {
 
+        const AP_GPS &gps = AP::gps();
         if (type_slope_stage != SLOPE_STAGE_FINAL) {
             type_slope_flags.post_stats = true;
             if (is_flying && (AP_HAL::millis()-last_flying_ms) > 3000) {
-                gcs().send_text(MAV_SEVERITY_CRITICAL, "Flare crash detected: speed=%.1f", (double)AP::gps().ground_speed());
+                gcs().send_text(MAV_SEVERITY_CRITICAL, "Flare crash detected: speed=%.1f", (double)gps.ground_speed());
             } else {
                 gcs().send_text(MAV_SEVERITY_INFO, "Flare %.1fm sink=%.2f speed=%.1f dist=%.1f",
                                   (double)height, (double)sink_rate,
-                                  (double)AP::gps().ground_speed(),
+                                  (double)gps.ground_speed(),
                                   (double)current_loc.get_distance(next_WP_loc));
             }
             
@@ -114,7 +116,7 @@ bool AP_Landing::type_slope_verify_land(const Location &prev_WP_loc, Location &n
             }
         }
 
-        if (AP::gps().ground_speed() < 3) {
+        if (gps.ground_speed() < 3) {
             // reload any airspeed or groundspeed parameters that may have
             // been set for landing. We don't do this till ground
             // speed drops below 3.0 m/s as otherwise we will change