Fix error when passing custom link style to SingleFieldList

[Slavvkko]

Using

const useSingleFieldListStyles = makeStyles({
  root: {
    display: "block",
    flexWrap: "unset",
    marginTop: 0,
    marginBottom: 0
  },
  link: {
    display: "block"
  }
});

const Demo = props => {
  const singleFieldListClasses = useSingleFieldListStyles();
  return (
    <SingleFieldList classes={singleFieldListClasses}>
      <TextField source="name"/>
    </SingleFieldList>
  );
};

Got an error in console:
Material-UI: the key link provided to the classes prop is not implemented in undefined.

Because styles in SingleFieldList doesn't have link class

[fzaninotto]

Thanks!