DQM/TrackingMonitorClient: add missing std::

The following is required for ROOT6.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #1960 from davidlt/add-missing-std-DQM-TrackingMonitorClient

Misc fixes -- DQM/TrackingMonitorClient: add missing std::