AC_WPNav: constify get_wp_destination

libraries/AC_WPNav/AC_WPNav.cpp

@@ -173,7 +173,8 @@ bool AC_WPNav::set_wp_destination(const Location& destination)
     return set_wp_destination(dest_neu, terr_alt);
 }
 
-bool AC_WPNav::get_wp_destination(Location& destination) {
+bool AC_WPNav::get_wp_destination(Location& destination) const
+{
     Vector3f dest = get_wp_destination();
     if (!AP::ahrs().get_origin(destination)) {
         return false;

libraries/AC_WPNav/AC_WPNav.h

@@ -104,7 +104,7 @@ public:
     // returns wp location using location class.
     // returns false if unable to convert from target vector to global
     // coordinates
-    bool get_wp_destination(Location& destination);
+    bool get_wp_destination(Location& destination) const;
 
     /// set_wp_destination waypoint using position vector (distance from ekf origin in cm)
     ///     terrain_alt should be true if destination.z is a desired altitude above terrain