[ros2] Export tinyxml2 directly from pluginlib-extras.cmake

[seanyen]

When I attempted to build nav2_rviz_plugins, I run into this CMake error:

CMake Error: install(EXPORT "nav2_rviz_plugins" ...) includes target "nav2_rviz_plugins" which requires target "tinyxml2_vendor" that is not in any export set.
CMake Generate step failed.  Build files cannot be regenerated correctly

After some digging, it is coming from this pluginlib-extras.cmake.

1. I either need to add IMPORTED to make tinyxml2_vendor as an interface imported library,
2. Or I need to directly define the pluginlib_TARGETS to be tinyxml2 which is the target exported from TinyXML2 project.

[seanyen]

@dirk-thomas This is ready for review and merge. Thanks.

[dirk-thomas]

@seanyen Please don't mention me on all PRs you create. Many repositories have dedicated maintainers which will review incoming tickets.

[seanyen]

@dirk-thomas Sorry for this one, I was mentioning you because it seems to be broken after this PR #189, so figured you might have better understanding of the issue I run into.

[dirk-thomas]

CI builds with only FastRTPS and no tests:

• Linux
• Linux-aarch64
• macOS
  • Unrelated CMake warnings
• Windows

[dirk-thomas]

Thanks for the patch.