Copter: implement get_wp() for follow

this allows the GCS to plot the position target

ArduCopter/mode.h

@@ -1142,6 +1142,7 @@ protected:
     const char *name4() const override { return "FOLL"; }
     uint32_t wp_distance() const override;
     int32_t wp_bearing() const override;
+    bool get_wp(Location_Class &loc) override;
 
     uint32_t last_log_ms;   // system time of last time desired velocity was logging
 };

ArduCopter/mode_follow.cpp

@@ -160,4 +160,16 @@ int32_t Copter::ModeFollow::wp_bearing() const
     return g2.follow.get_bearing_to_target() * 100;
 }
 
+/*
+  get target position for mavlink reporting
+ */
+bool Copter::ModeFollow::get_wp(Location_Class &loc)
+{
+    float dist = g2.follow.get_distance_to_target();
+    float bearing = g2.follow.get_bearing_to_target();
+    loc = copter.current_loc;
+    location_update(loc, bearing, dist);
+    return true;
+}
+
 #endif // MODE_FOLLOW_ENABLED == ENABLED