direct: Fix undefined ConfigVariableBool name in ClockDelta

direct/src/distributed/ClockDelta.py

@@ -1,7 +1,7 @@
 # ClockDelta provides the ability to use clock synchronization for
 # distributed objects
 
-from panda3d.core import ClockObject
+from panda3d.core import ClockObject, ConfigVariableBool
 from direct.directnotify import DirectNotifyGlobal
 from direct.showbase import DirectObject
 import math