It is possible that the water is coming into the roof and is exiting below, where the soffit and the light are located. This is unlikely and I suspect that the water is dripping over the roof and onto the light fixture. If this is the case, there are two possible reasons. First, the water that is supposed to be dripping into the eave is actually dripping between the eave and the siding and is eventually ending up at the light fixture, or the water may be following an electrical wire that runs from the siding to the light. The first thing that you have to do is determine where the water is coming from and how it is reaching the light fixture. A simple trick to find the answer to this question is to take some colored chalk and spread it on the bottom side of the soffit around the light fixture. After the next rainfall investigate the chalk. If the chalk has not been disturbed at all, then the water leakage is coming from the inside. If some of the chalk is missing, then it will leave a clear path and indicate where the water is coming from. If the water is dripping between the eave and the siding, a simple drip-edge flashing at the edge of the roof should solve the problem. Another possibility is to apply a coat of caulking around the top of the light fixture that will create a water barrier. It will not prevent the water from reaching the light fixture, but it will prevent it from entering it.