Port back from ROOT6 IB for commonality (DQM)

[wmtan]

This PR ports back simple changes made in CMSSW_7_4_ROOT6_X in the DQM L2 category that are desirable or harmless for CMSSW_7_4_X, in order to increase code commonality. All affected files will be identical in the two releases after this PR is merged. Changes were successfully tested with the short relval matrix.
The changes are:

1. Addition of a const qualifier needed in CMSSW_7_4_ROOT6_X
2. Addition of a missing std:: qualifier
3. Removal of an unnecessary include.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_4_X.

Port back from ROOT6 IB for commonality (DQM)

It involves the following packages:

DQM/Physics
DQMServices/Core
DQMServices/StreamerIO

@cmsbuild, @danduggan, @nclopezo, @deguio can you please review it and eventually sign? Thanks.
@rappoccio, @barvic this is something you requested to watch as well.
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@Degano you are the release manager for this.
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