msgs.cc: add missing <array> include

[scpeters]

🦟 Bug fix

Fixes osrf/homebrew-simulation#2179

Summary

There is a reported build failure on some versions of macOS in osrf/homebrew-simulation#2179, and I observed a failed bottle build on macOS Monterey:

https://build.osrfoundation.org/job/generic-release-homebrew_triggered_bottle_builder/1088/label=osx_monterey/

There is a complaint about msgs.cc:

gazebo/msgs/msgs.cc:2023:34: error: implicit instantiation of undefined template 'std::array<std::string, 2>'
std::array<std::string, 2> dimensions = {{"horizontal", "vertical"}};
^

The solution is to include the <array> header file.

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: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[traversaro]

Please ignore conda-forge CI failures, they are not related to the PR and will be solved by #3287 .

[scpeters]

I'm not sure why the abi-checker jobs are failing too; I'll look into that. I'll merge this anyway since it doesn't affect API/ABI