/usr/include/geos/geom/prep/ is in libgeos-dev 3.2.2-3ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/geos/geom/prep/AbstractPreparedPolygonContains.h
/usr/include/geos/geom/prep/BasicPreparedGeometry.h
/usr/include/geos/geom/prep/PreparedGeometry.h
/usr/include/geos/geom/prep/PreparedGeometryFactory.h
/usr/include/geos/geom/prep/PreparedLineString.h
/usr/include/geos/geom/prep/PreparedLineStringIntersects.h
/usr/include/geos/geom/prep/PreparedPoint.h
/usr/include/geos/geom/prep/PreparedPolygon.h
/usr/include/geos/geom/prep/PreparedPolygonContains.h
/usr/include/geos/geom/prep/PreparedPolygonContainsProperly.h
/usr/include/geos/geom/prep/PreparedPolygonCovers.h
/usr/include/geos/geom/prep/PreparedPolygonIntersects.h
/usr/include/geos/geom/prep/PreparedPolygonPredicate.h