bitbake: event: fix resetting class handlers object

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: f12c738d3dc1f0fd105d457385511440024bffab)

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