CondCore/CondDB: add missing `TBufferFile.h` include in Serialization.cc

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #2974 from davidlt/CondCore-CondDB-add-missing-TBufferFile.h

CondCore/CondDB: add missing `TBufferFile.h` include in Serialization.cc