Fix type issue with creation of LabelHeader

[nbarraille]

Missing generic declaration in HeaderCell type creates typescript error when doing the following:

export function createDataTableHeaderCell<Item>(): HeaderLabel<Item, TablePlugins<Item>> {
    return (cell: HeaderCell<Item, TablePlugins<Item>>) =>
      createRender(MyHeaderCellComponent<Item>, {
        props: cell.props(),
      });
  }

==>

Type '(cell: HeaderCell<Item, TablePlugins<Item>>) => ComponentRenderConfig<DataTableHeaderCell<Item>>' is not assignable to type 'HeaderLabel<Item, TablePlugins<Item>>'.