AR_WPNav: avoid using struct Location

clang reports this could be a problem when compiling under some EABIs.  Remove it from most places as it is just noise, replace with class where we want to avoid including Location.h

libraries/AR_WPNav/AR_WPNav.cpp

@@ -199,7 +199,7 @@ void AR_WPNav::set_nudge_speed_max(float nudge_speed_max)
 
 // set desired location and (optionally) next_destination
 // next_destination should be provided if known to allow smooth cornering
-bool AR_WPNav::set_desired_location(const struct Location& destination, Location next_destination)
+bool AR_WPNav::set_desired_location(const Location& destination, Location next_destination)
 {
     // re-initialise if inactive, previous destination has been interrupted or different controller was used
     if (!is_active() || !_reached_destination || (_nav_control_type != NavControllerType::NAV_SCURVE)) {

libraries/AR_WPNav/AR_WPNav_OA.cpp

@@ -137,7 +137,7 @@ void AR_WPNav_OA::update(float dt)
 
 // set desired location and (optionally) next_destination
 // next_destination should be provided if known to allow smooth cornering
-bool AR_WPNav_OA::set_desired_location(const struct Location& destination, Location next_destination)
+bool AR_WPNav_OA::set_desired_location(const Location& destination, Location next_destination)
 {
     const bool ret = AR_WPNav::set_desired_location(destination, next_destination);