MuonAnalysis/MomentumScaleCalibration: add missing `std::`

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #2969 from davidlt/MuonAnalysis-MomentumScaleCalibration-add-missing-std

MuonAnalysis/MomentumScaleCalibration: add missing `std::`