/usr/include/healpix_cxx/ is in libhealpix-cxx-dev 3.30.0-6.
This file is owned by root:root, with mode 0o755.
..
/usr/include/healpix_cxx/alloc_utils.h
/usr/include/healpix_cxx/alm.h
/usr/include/healpix_cxx/alm_fitsio.h
/usr/include/healpix_cxx/alm_healpix_tools.h
/usr/include/healpix_cxx/alm_powspec_tools.h
/usr/include/healpix_cxx/arr.h
/usr/include/healpix_cxx/datatypes.h
/usr/include/healpix_cxx/error_handling.h
/usr/include/healpix_cxx/fitshandle.h
/usr/include/healpix_cxx/healpix_base.h
/usr/include/healpix_cxx/healpix_data_io.h
/usr/include/healpix_cxx/healpix_map.h
/usr/include/healpix_cxx/healpix_map_fitsio.h
/usr/include/healpix_cxx/healpix_tables.h
/usr/include/healpix_cxx/lsconstants.h
/usr/include/healpix_cxx/math_utils.h
/usr/include/healpix_cxx/moc.h
/usr/include/healpix_cxx/moc_fitsio.h
/usr/include/healpix_cxx/moc_query.h
/usr/include/healpix_cxx/pointing.h
/usr/include/healpix_cxx/powspec.h
/usr/include/healpix_cxx/powspec_fitsio.h
/usr/include/healpix_cxx/rangeset.h
/usr/include/healpix_cxx/rotmatrix.h
/usr/include/healpix_cxx/safe_cast.h
/usr/include/healpix_cxx/sharp.h
/usr/include/healpix_cxx/sharp_geomhelpers.c
/usr/include/healpix_cxx/sharp_geomhelpers.h
/usr/include/healpix_cxx/sharp_lowlevel.h
/usr/include/healpix_cxx/sse_utils_cxx.h
/usr/include/healpix_cxx/string_utils.h
/usr/include/healpix_cxx/vec3.h
/usr/include/healpix_cxx/wigner.h
/usr/include/healpix_cxx/xcomplex.h