/usr/include/ is in libnacl-dev 20110221-5.
This file is owned by root:root, with mode 0o755.
..
/usr/include/nacl/
/usr/include/nacl/crypto_auth.h
/usr/include/nacl/crypto_auth_hmacsha256.h
/usr/include/nacl/crypto_auth_hmacsha512256.h
/usr/include/nacl/crypto_box.h
/usr/include/nacl/crypto_box_curve25519xsalsa20poly1305.h
/usr/include/nacl/crypto_core_hsalsa20.h
/usr/include/nacl/crypto_core_salsa20.h
/usr/include/nacl/crypto_core_salsa2012.h
/usr/include/nacl/crypto_core_salsa208.h
/usr/include/nacl/crypto_hash.h
/usr/include/nacl/crypto_hash_sha256.h
/usr/include/nacl/crypto_hash_sha512.h
/usr/include/nacl/crypto_hashblocks.h
/usr/include/nacl/crypto_hashblocks_sha256.h
/usr/include/nacl/crypto_hashblocks_sha512.h
/usr/include/nacl/crypto_int16.h
/usr/include/nacl/crypto_int32.h
/usr/include/nacl/crypto_int64.h
/usr/include/nacl/crypto_int8.h
/usr/include/nacl/crypto_onetimeauth.h
/usr/include/nacl/crypto_onetimeauth_poly1305.h
/usr/include/nacl/crypto_scalarmult_curve25519.h
/usr/include/nacl/crypto_secretbox.h
/usr/include/nacl/crypto_secretbox_xsalsa20poly1305.h
/usr/include/nacl/crypto_sign.h
/usr/include/nacl/crypto_sign_edwards25519sha512batch.h
/usr/include/nacl/crypto_stream.h
/usr/include/nacl/crypto_stream_aes128ctr.h
/usr/include/nacl/crypto_stream_salsa20.h
/usr/include/nacl/crypto_stream_salsa2012.h
/usr/include/nacl/crypto_stream_salsa208.h
/usr/include/nacl/crypto_stream_xsalsa20.h
/usr/include/nacl/crypto_uint16.h
/usr/include/nacl/crypto_uint32.h
/usr/include/nacl/crypto_uint64.h
/usr/include/nacl/crypto_uint8.h
/usr/include/nacl/crypto_verify_16.h
/usr/include/nacl/crypto_verify_32.h
/usr/include/nacl/randombytes.h