/usr/lib/grass74/include/grass/iostream/ is in grass-dev 7.4.0-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/grass74/include/grass/iostream/ami.h
/usr/lib/grass74/include/grass/iostream/ami_config.h
/usr/lib/grass74/include/grass/iostream/ami_sort.h
/usr/lib/grass74/include/grass/iostream/ami_sort_impl.h
/usr/lib/grass74/include/grass/iostream/ami_stream.h
/usr/lib/grass74/include/grass/iostream/embuffer.h
/usr/lib/grass74/include/grass/iostream/empq.h
/usr/lib/grass74/include/grass/iostream/empq_adaptive.h
/usr/lib/grass74/include/grass/iostream/empq_adaptive_impl.h
/usr/lib/grass74/include/grass/iostream/empq_impl.h
/usr/lib/grass74/include/grass/iostream/imbuffer.h
/usr/lib/grass74/include/grass/iostream/mem_stream.h
/usr/lib/grass74/include/grass/iostream/minmaxheap.h
/usr/lib/grass74/include/grass/iostream/mm.h
/usr/lib/grass74/include/grass/iostream/mm_utils.h
/usr/lib/grass74/include/grass/iostream/pqheap.h
/usr/lib/grass74/include/grass/iostream/queue.h
/usr/lib/grass74/include/grass/iostream/quicksort.h
/usr/lib/grass74/include/grass/iostream/replacementHeap.h
/usr/lib/grass74/include/grass/iostream/replacementHeapBlock.h
/usr/lib/grass74/include/grass/iostream/rtimer.h