Fix CMake warning in systems/plants on minimal builds
Fix CMake warning in systems/plants on minimal builds

billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake