AnalysisTop: Implementing a workaround for ROOT 6.24 bug

This is an update that is needed for AnalysisTop code when custom event savers are used. Without this change, the ROOT dictionary cannot be properly buillt.

It was identified that this is a bug in ROOT: https://github.com/root-project/root/issues/8469

To make it obvious, the weird part here is that the code is already inside top namespace so the additional top:: should be superfluous, but apparently is needed for the ROOT dictionary building.

Fixes ATLASG-1596