bitbake: event: fix resetting class handlers object

If you don't explicitly specify to use a global variable when doing an
assignment, you will be setting a local variable instead, which means
this function wasn't working at all. It explains some odd behaviour we
have seen in the layer index where event handlers were sometimes
bleeding into other contexts where they should not have been.

(Bitbake rev: ac45ea848901b0f6cd23087b662dde8ce9cd807e)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/event.py

@@ -55,6 +55,7 @@ def get_class_handlers():
     return _handlers
 
 def set_class_handlers(h):
+    global _handlers
     _handlers = h
 
 def clean_class_handlers():