Add missing include of TObjString.h

Merged Marco Clemencic requested to merge fixes-for-new-root into master

@cvazquez, this fixes 94 of the errors in lcg-dev* slots we were discussing this morning.

I labelled it only for those slots, but it's backward compatible, so I'll let you decide when to test it in other slots and merge.

Edited by Carlos Vazquez Sierra

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