AC_WPNav: don't link AC_Avoid on plane

saves about 7k of flash

libraries/AC_WPNav/AC_Loiter.cpp

@@ -1,5 +1,6 @@
 #include <AP_HAL/AP_HAL.h>
 #include "AC_Loiter.h"
+#include <AP_Vehicle/AP_Vehicle_Type.h>
 
 extern const AP_HAL::HAL& hal;
 
@@ -274,6 +275,7 @@ void AC_Loiter::calc_desired_velocity(float nav_dt, bool avoidance_on)
         desired_vel.y = desired_vel.y * gnd_speed_limit_cms / horizSpdDem;
     }
 
+#if !APM_BUILD_TYPE(APM_BUILD_ArduPlane)
     if (avoidance_on) {
         // Limit the velocity to prevent fence violations
         // TODO: We need to also limit the _desired_accel
@@ -284,6 +286,7 @@ void AC_Loiter::calc_desired_velocity(float nav_dt, bool avoidance_on)
             desired_vel = Vector2f{avoidance_vel_3d.x, avoidance_vel_3d.y};
         }
     }
+#endif // !APM_BUILD_ArduPlane
 
     // get loiters desired velocity from the position controller where it is being stored.
     Vector2p target_pos = _pos_control.get_pos_target_cm().xy();