/usr/include/Inventor/Xt/ is in inventor-dev 2.1.5-10-16ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/Inventor/Xt/SoXt.h
/usr/include/Inventor/Xt/SoXtClipboard.h
/usr/include/Inventor/Xt/SoXtComponent.h
/usr/include/Inventor/Xt/SoXtCursors.h
/usr/include/Inventor/Xt/SoXtDirectionalLightEditor.h
/usr/include/Inventor/Xt/SoXtGLWidget.h
/usr/include/Inventor/Xt/SoXtIcons.h
/usr/include/Inventor/Xt/SoXtLightSliderSet.h
/usr/include/Inventor/Xt/SoXtMaterialEditor.h
/usr/include/Inventor/Xt/SoXtMaterialList.h
/usr/include/Inventor/Xt/SoXtMaterialSliderSet.h
/usr/include/Inventor/Xt/SoXtPrintDialog.h
/usr/include/Inventor/Xt/SoXtRenderArea.h
/usr/include/Inventor/Xt/SoXtResource.h
/usr/include/Inventor/Xt/SoXtSliderSet.h
/usr/include/Inventor/Xt/SoXtSliderSetBase.h
/usr/include/Inventor/Xt/SoXtTransformSliderSet.h
/usr/include/Inventor/Xt/devices/
/usr/include/Inventor/Xt/devices/SoXtDevice.h
/usr/include/Inventor/Xt/devices/SoXtInputFocus.h
/usr/include/Inventor/Xt/devices/SoXtKeyboard.h
/usr/include/Inventor/Xt/devices/SoXtMouse.h
/usr/include/Inventor/Xt/devices/SoXtSpaceball.h
/usr/include/Inventor/Xt/viewers/
/usr/include/Inventor/Xt/viewers/SoXtConstrainedViewer.h
/usr/include/Inventor/Xt/viewers/SoXtExaminerViewer.h
/usr/include/Inventor/Xt/viewers/SoXtFlyViewer.h
/usr/include/Inventor/Xt/viewers/SoXtFullViewer.h
/usr/include/Inventor/Xt/viewers/SoXtPlaneViewer.h
/usr/include/Inventor/Xt/viewers/SoXtViewer.h
/usr/include/Inventor/Xt/viewers/SoXtWalkViewer.h