Make testlibusb.c compatible with libusb < 1.0.23

tests/testlibusb.c

@@ -218,7 +218,10 @@ static void print_device(libusb_device *dev, libusb_device_handle *handle)
 
     case LIBUSB_SPEED_SUPER:    speed = "5G"; break;
 
+#if defined(LIBUSB_API_VERSION) && (LIBUSB_API_VERSION >= 0x01000107)
+
     case LIBUSB_SPEED_SUPER_PLUS:   speed = "10G"; break;
+#endif
 
     default:            speed = "Unknown";
     }