Add missing <cstddef> includes for size_t

[guitargeek]

PR description:

To make cmssw compile with gcc 11.

PR validation:

Building CMSSW with gcc 11 gives errors without this PR, but succeeds with this PR.

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-33820/22819

• This PR adds an extra 16KB to repository

[cmsbuild]

A new Pull Request was created by @guitargeek (Jonas Rembser) for master.

It involves the following packages:

DataFormats/HcalDigi
DataFormats/SiPixelDigi

@cmsbuild, @civanch, @mdhildreth can you please review it and eventually sign? Thanks.
@makortel, @rovere, @VinInn, @OzAmram, @dkotlins, @ferencek, @mariadalfonso, @abdoulline, @tvami this is something you requested to watch as well.
@silviodonato, @dpiparo, @qliphy you are the release manager for this.

cms-bot commands are listed here

[qliphy]

please test

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-9340a3/15265/summary.html
COMMIT: c502b62
CMSSW: CMSSW_12_0_X_2021-05-23-2300/slc7_amd64_gcc900
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/33820/15265/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

Summary:

• No significant changes to the logs found
• Reco comparison results: 4 differences found in the comparisons
• DQMHistoTests: Total files compared: 37
• DQMHistoTests: Total histograms compared: 2650486
• DQMHistoTests: Total failures: 7
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 2650457
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 36 files compared)
• Checked 155 log files, 37 edm output root files, 37 DQM output files
• TriggerResults: no differences found

[VinInn]

I pretend that the correct action is to change size_t in uint32_t

[civanch]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @silviodonato, @dpiparo, @qliphy (and backports should be raised in the release meeting by the corresponding L2)

[civanch]

@VinInn , this is not the only place where size_t is used. I do not know arguments pro and contr but if size_t should be substituted by uint32_t, then this should be a campaign to substitute everywhere.

[qliphy]

+1