(Reuters) - Private equity firm Thoma Bravo LLC has approached Symantec Corp (SYMC.O) to express interest in acquiring the Norton antivirus software maker, people familiar with the matter told Reuters on Tuesday. A deal could be the largest leveraged buyout this year, based on Symantec’s market value of about $15 billion and total debt of approximately $5 billion. It would come as Symantec seeks to regain its footing in a crowded cyber security market following a string of acquisitions, including the $4.65 billion purchase of cyber security firm Blue Coat Inc in 2016.