/usr/include/ is in libx32stdc++-6-dev 6.3.0-18+deb9u1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/x86_64-linux-gnu/
/usr/include/x86_64-linux-gnu/c++/
/usr/include/x86_64-linux-gnu/c++/6/
/usr/include/x86_64-linux-gnu/c++/6/x32/
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/atomic_word.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/basic_file.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/c++allocator.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/c++config.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/c++io.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/c++locale.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/cpu_defines.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/ctype_base.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/ctype_inline.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/cxxabi_tweaks.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/error_constants.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/extc++.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/gthr-default.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/gthr-posix.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/gthr-single.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/gthr.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/messages_members.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/opt_random.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/os_defines.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/stdc++.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/stdtr1c++.h
/usr/include/x86_64-linux-gnu/c++/6/x32/bits/time_members.h
/usr/include/x86_64-linux-gnu/c++/6/x32/ext/
/usr/include/x86_64-linux-gnu/c++/6/x32/ext/opt_random.h