AP_Mission: allow NAV_VTOL_TAKEOFF in is_takoff_next()

libraries/AP_Mission/AP_Mission.cpp

@@ -161,6 +161,7 @@ bool AP_Mission::is_takeoff_next(uint16_t cmd_index)
         }
         switch (cmd.id) {
         // any of these are considered a takeoff command:
+        case MAV_CMD_NAV_VTOL_TAKEOFF:
         case MAV_CMD_NAV_TAKEOFF:
         case MAV_CMD_NAV_TAKEOFF_LOCAL:
             return true;