Rolling Stone Archive PhotoI heard whispers last week that computer/Internet hotshot Mike Slade had a secret guest planned for his 50th birthday party, Saturday at The Showbox. Slade confirmed today that the noise coming out of the First Avenue club was indeed none other than Steve Winwood.”I think most people were surprised,” he said. “He was so good. He played a bunch of Traffic tunes, solo, unbelievable stuff.”I have a friend who is convinced Def Leppard will play her 30th. For my 50th, I’m calling Los Del Rio: