[Windows][ros2] fix multi_camera_plugin on windows

[jnoyola]

Another update so the latest changes are compatible with Windows. These changes should have no effect on other systems.

• Link multi_camera_plugin with the gazebo_sensors import library.
• Set BUILDING_DLL to correctly assign the GAZEBO_VISIBLE attribute for export
  • Note this is also used by Gazebo when building the built-in plugins

By the way, what is the method that gets these changes into the version branches (e.g. dashing)? How often/soon does that happen?

[chapulina]

Thank you for the PR, the changes look good to me.

By the way, what is the method that gets these changes into the version branches (e.g. dashing)? How often/soon does that happen?

I've been asking that backwards compatible PRs be targetted at dashing and from time to time someone cherry-picks them to ros2 as needed. Some of the advantages being:

• CI is run automatically for that branch
• It's usually easier to forward-port features than backport them, because there's no concern of API / ABI breakage.

So with that in mind, if you could open a PR against dashing, I'll merge it if CI is clear, and then merge this one as well.

Thanks!

Note to self: document this on the contribution guide.

[jnoyola]

Got it! Here's #999 on dashing.