/usr/include/thunarx-2/ is in libthunarx-2-dev 1.6.15-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/thunarx-2/thunarx/
/usr/include/thunarx-2/thunarx/thunarx-config.h
/usr/include/thunarx-2/thunarx/thunarx-file-info.h
/usr/include/thunarx-2/thunarx/thunarx-menu-provider.h
/usr/include/thunarx-2/thunarx/thunarx-preferences-provider.h
/usr/include/thunarx-2/thunarx/thunarx-property-page-provider.h
/usr/include/thunarx-2/thunarx/thunarx-property-page.h
/usr/include/thunarx-2/thunarx/thunarx-provider-factory.h
/usr/include/thunarx-2/thunarx/thunarx-provider-plugin.h
/usr/include/thunarx-2/thunarx/thunarx-renamer-provider.h
/usr/include/thunarx-2/thunarx/thunarx-renamer.h
/usr/include/thunarx-2/thunarx/thunarx.h