The fifth amendment doesn't even apply here. It's a first amendment issue. As a member of the press, his right to report on anything, including secret classified documents, cannot be curtailed. Forcing him to reveal his source falls under that protection because it limits his ability and the ability of other reporters going forward to receive similar offers of assistance from sources. It would be murkier if he had signed any sort of agreement to gain clearance to the documents, but he didn't.
If the situation were different and Risen had been found to have classified documents during a legal search, then he could be compelled to reveal where he got those documents so long as he wasn't incriminating himself. But it's not, he's an member of the press and the first amendment protects the press's right to keep their sources secret.