/usr/include/signon-qt5/ is in libsignon-qt5-dev 8.59-1+b2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/signon-qt5/SignOn/
/usr/include/signon-qt5/SignOn/AuthService
/usr/include/signon-qt5/SignOn/AuthSession
/usr/include/signon-qt5/SignOn/Error
/usr/include/signon-qt5/SignOn/Identity
/usr/include/signon-qt5/SignOn/IdentityInfo
/usr/include/signon-qt5/SignOn/SessionData
/usr/include/signon-qt5/SignOn/authservice.h
/usr/include/signon-qt5/SignOn/authsession.h
/usr/include/signon-qt5/SignOn/identity.h
/usr/include/signon-qt5/SignOn/identityinfo.h
/usr/include/signon-qt5/SignOn/libsignoncommon.h
/usr/include/signon-qt5/SignOn/sessiondata.h
/usr/include/signon-qt5/SignOn/signon.h
/usr/include/signon-qt5/SignOn/signonerror.h