Use spdlog_vendor in package.xml

[azeey]

🦟 Bug fix

Summary

spdlog is not being properly found on ROS windows builds. Example: https://ci.ros2.org/view/nightly/job/nightly_win_rel/3158/cmake/new/

Per @clalancette's recommendation, this patch changes the package.xml entry to spdlog_vendor.

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.

[azeey]

Yup, you're right. This should be done in gz_utils_vendor. I've updated the generation script https://github.com/gazebo-tooling/gz_vendor/pull/9 and I've approved https://github.com/gazebo-release/gz_utils_vendor/pull/6