BERLIN (Reuters) - KKR has become the biggest shareholder of German media group Axel Springer , paying 2.9 billion euros (£2.6 billion)for a 43.54% stake, the U.S. private equity firm said on Monday.The stakebuilding by KKR in concert with Friede Springer, the widow of the company's founder, and CEO Mathias Doepfner is aimed at taking the publisher private by the end of the year or in the first quarter of 2020.KKR achieved a 42.5% stake with its tender to buy out minority shareholders and..