Generator: Add missing std namespace to string arguments (#242)

[scpeters]

🦟 Bug fix

Backport #242 to ign-msgs1

Summary

Fix homebrew build:

• https://build.osrfoundation.org/job/ignition_msgs-ci-ign-msgs1-homebrew-amd64/33/

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Use rebase and merge

[scpeters]

the windows build needs gazebo-tooling/gazebodistro#114

[scpeters]

I added an ign-msgs1.yaml file in gazebo-tooling/gazebodistro#114 to try to fix the windows build, but this is still failing due to issues with popen. I'll spend a few more minutes trying to fix the windows build but will override the windows CI check to merge this if I don't see a quick fix since the issue is pre-existing, and this PR fixes the macOS build.

[chapulina]

will override the windows CI check to merge this if I don't see a quick fix since the issue is pre-existing

SGTM, just make it not required for the ign-msgs[0-1] branch rule and remove it from gazebo-tooling/release-tools#398 so we don't need to manually override it every time.