Add missing include to fix build problem

[wmtan]

This trivial PR adds a missing include to fix a build error in CMSSW_7_4_ROOT5_X.
Since the header is also missing in 7_4_X, and for code commonality, this fix should go into 7_4_X as well, since it is just an accident that there is no build error due to this in 7_4_X.

[cmsbuild]

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

Add missing include to fix build problem

It involves the following packages:

PhysicsTools/HepMCCandAlgos

@cmsbuild, @vadler, @monttj can you please review it and eventually sign? Thanks.
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@Degano you are the release manager for this.
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