showbase: Fix missing ClockObject import in JobManager

Regression in eefcae7b05aef7628b691d8601f8d7d9d0916cb3

direct/src/showbase/JobManager.py

@@ -1,4 +1,4 @@
-from panda3d.core import ConfigVariableBool, ConfigVariableDouble
+from panda3d.core import ConfigVariableBool, ConfigVariableDouble, ClockObject
 from direct.directnotify.DirectNotifyGlobal import directNotify
 from direct.task.TaskManagerGlobal import taskMgr
 from direct.showbase.Job import Job