Small fix for errors in UE4 natvis file with VS 2015

As of VS 2015, the UE4 natvis file for debugger visualization of UE4 types doesn’t work properly for a few types (TMap/TSet in particular).
It’s a very simple fix, I’ve made the modifications here.

Just posting this here as a request to have the file updated, since it isn’t part of the Github repo so I can’t submit it there.