oeqa/core/decorator/__init__.py: set metaclass to ABCMeta
OETestFilter is a subclass of OETestDecorator. It wants to make
use of @abstractmethod decorator. But such decorator requires
metaclass to be ABCMeta to have effect. So add it now to achieve
the designed behaviour.
Comments from python's manual:
"""
Using this decorator requires that the class's metaclass is ABCMeta
or is derived from it.
"""
Signed-off-by: Chen Qi <Qi.Chen@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
