/usr/include/ is in libsignal-protocol-c-dev 2.3.1+git20171007-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/signal/
/usr/include/signal/curve.h
/usr/include/signal/device_consistency.h
/usr/include/signal/fingerprint.h
/usr/include/signal/group_cipher.h
/usr/include/signal/group_session_builder.h
/usr/include/signal/hkdf.h
/usr/include/signal/key_helper.h
/usr/include/signal/protocol.h
/usr/include/signal/ratchet.h
/usr/include/signal/sender_key.h
/usr/include/signal/sender_key_record.h
/usr/include/signal/sender_key_state.h
/usr/include/signal/session_builder.h
/usr/include/signal/session_cipher.h
/usr/include/signal/session_pre_key.h
/usr/include/signal/session_record.h
/usr/include/signal/session_state.h
/usr/include/signal/signal_protocol.h
/usr/include/signal/signal_protocol_types.h