The GCC version on the buildbot does not support attribute on
a class member, resulting in the following warning:
NOD_node_declaration.hh:577:42: warning: ‘maybe_unused’ attribute ignored [-Wattributes]
Use the `UNUSED_VARS` macro instead to solve the original warning
about member being unused in release builds without introducing
a warning when using older compiler.
Pull Request: https://projects.blender.org/blender/blender/pulls/131974
630c3ce94e