/usr/include/stxxl/bits/mng/ is in libstxxl-dev 1.3.1-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/stxxl/bits/mng/adaptor.h
/usr/include/stxxl/bits/mng/bid.h
/usr/include/stxxl/bits/mng/block_alloc.h
/usr/include/stxxl/bits/mng/block_alloc_interleaved.h
/usr/include/stxxl/bits/mng/block_prefetcher.h
/usr/include/stxxl/bits/mng/buf_istream.h
/usr/include/stxxl/bits/mng/buf_ostream.h
/usr/include/stxxl/bits/mng/buf_writer.h
/usr/include/stxxl/bits/mng/config.h
/usr/include/stxxl/bits/mng/diskallocator.h
/usr/include/stxxl/bits/mng/mng.h
/usr/include/stxxl/bits/mng/prefetch_pool.h
/usr/include/stxxl/bits/mng/read_write_pool.h
/usr/include/stxxl/bits/mng/typed_block.h
/usr/include/stxxl/bits/mng/write_pool.h