/usr/include/nautilus/libnautilus-extension/ is in libnautilus-extension-dev 1:3.4.1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/nautilus/libnautilus-extension/nautilus-column-provider.h
/usr/include/nautilus/libnautilus-extension/nautilus-column.h
/usr/include/nautilus/libnautilus-extension/nautilus-extension-types.h
/usr/include/nautilus/libnautilus-extension/nautilus-file-info.h
/usr/include/nautilus/libnautilus-extension/nautilus-info-provider.h
/usr/include/nautilus/libnautilus-extension/nautilus-location-widget-provider.h
/usr/include/nautilus/libnautilus-extension/nautilus-menu-item.h
/usr/include/nautilus/libnautilus-extension/nautilus-menu-provider.h
/usr/include/nautilus/libnautilus-extension/nautilus-menu.h
/usr/include/nautilus/libnautilus-extension/nautilus-property-page-provider.h
/usr/include/nautilus/libnautilus-extension/nautilus-property-page.h