Update GameListCtrl.cpp

The file sizes were not being displayed correctly. The previous code was not working as intended. I have come up with a solution that spits out the correct file sizes (I think). It is probably not the most efficient code ever, so feel free, please, to optimize it. I'm very wary of the (double) casting of the u64s, but I don't know any better way to get the remainder in fraction form, and our precision isn't very high anyways.