Add missing iostream include in TauAnalysis/MCEmbeddingTools

The following is needed for ROOT6.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #1669 from davidlt/add-missing-iostream-incl-TauAnalysis-MCEmbeddingTools-70X

Add missing iostream include in TauAnalysis/MCEmbeddingTools