Fix change_ordering=True for global average pool

[Vozf]

Currently change_ordering=True doesn't conver global average pool. With the addition of argument axis to the target function target_func will fall under this condition https://github.com/nerox8664/onnx2keras/blob/b5d77c0920eeefef86ea309d7776452a85ec454b/onnx2keras/converter.py#L242 And later conversion here. Sorry didn't test on master, but tested on the latest pypi release and structure is a little changed in a new code https://github.com/nerox8664/onnx2keras/blob/b5d77c0920eeefef86ea309d7776452a85ec454b/onnx2keras/converter.py#L271