L2SS-392: Add missing import

tangostationcontrol/tangostationcontrol/devices/unb2.py

@@ -21,6 +21,7 @@ from tangostationcontrol.common.entrypoint import entry
 from tangostationcontrol.clients.attribute_wrapper import attribute_wrapper
 from tangostationcontrol.devices.opcua_device import opcua_device
 from tangostationcontrol.common.lofar_logging import device_logging_to_python, log_exceptions
+from tangostationcontrol.devices.device_decorators import only_when_on
 
 import numpy