API: _get_ndarray_c_version imported but not added to __all__

`_get_ndarray_c_version` is [imported](https://github.com/numpy/numpy/blob/v1.18.1/numpy/core/multiarray.py#L20) from `_multiarray_umath` but not added to `__all__`. I am not sure what would be more correct: remove it from the import or add it to `__all__`