protected function _renderFiltersBefore()
    {
       $this->getSelect()->joinLeft(
                        array('stock_item' => $this->getTable('cataloginventory_stock_item')),
                'main_table.item_id  = stock_item.item_id', array('product_id'=> 'product.sku'))
                ->joinLeft(
                        array('product' => $this->getTable('catalog_product_entity')),
                        'stock_item.product_id = product.entity_id', array('product_sku' => 'product.sku')
        );

        parent::_renderFiltersBefore();
    }

<column name="sku">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="filter" xsi:type="string">text</item>
                    <item name="label" xsi:type="string" translate="true">Sku</item>
                </item>
            </argument>
        </column>  <column name="sku">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="filter" xsi:type="string">text</item>
                    <item name="label" xsi:type="string" translate="true">Sku</item>
                </item>
            </argument>
        </column>