Set channel in abstract ProcessLogEvent

This doesn't really do anything but it was changed on master in
dd7d2e1ad332593d760db5dd4431e5527148a409 and applying it here
also makes diffing the branches easier.

supervisor/events.py

@@ -19,6 +19,7 @@ class Event:
 
 class ProcessLogEvent(Event):
     """ Abstract """
+    channel = None
     def __init__(self, process, pid, data):
         self.process = process
         self.pid = pid