Refs #9977. Add const to DataReader::guid() (backport #2590)

[mergify]

This is an automatic backport of pull request #2590 done by Mergify.
Cherry-pick of e8581db has failed:

On branch mergify/bp/2.1.x/pr-2590
Your branch is up to date with 'origin/2.1.x'.

You are currently cherry-picking commit e8581dbe3.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/cpp/fastdds/subscriber/DataReader.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   include/fastdds/dds/subscriber/DataReader.hpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com

[MiguelCompany]

@pablogs9 Would you mind solving conflicts here?

[pablogs9]

Done! Thanks!

[MiguelCompany]

LGTM. Thanks!