Add missing std::

[wmtan]

This trivial pull request adds a missing std:: qualifier, and, in doing so, removes an unnecessary difference between CMSSW_7_4_X and CMSSW_7_4_ROOT6_X.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_5_X.

Add missing std::

It involves the following packages:

Fireworks/Core

@cmsbuild, @alja, @Dr15Jones, @ktf, @nclopezo can you please review it and eventually sign? Thanks.
@alja this is something you requested to watch as well.
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@nclopezo you are the release manager for this.
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