Fix FileSystemPosix for OS X

This should fix #3182, but it needs testing. In particular, assuming it builds, it would be good to check that the times listed in the load game dialog match the times reported by the Finder (it's called the Finder, right? I don't even know) for the save files.

/cc @eloquentmess
Do we really need sub-second resolution here? Or can we just get rid of this line and have the same code on all platforms?