/usr/include/KF5/KWindowSystem/ is in libkf5windowsystem-dev 5.18.0-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/KF5/KWindowSystem/KKeyServer
/usr/include/KF5/KWindowSystem/KSelectionOwner
/usr/include/KF5/KWindowSystem/KSelectionWatcher
/usr/include/KF5/KWindowSystem/KStartupInfo
/usr/include/KF5/KWindowSystem/KUserTimestamp
/usr/include/KF5/KWindowSystem/KWindowEffects
/usr/include/KF5/KWindowSystem/KWindowInfo
/usr/include/KF5/KWindowSystem/KWindowSystem
/usr/include/KF5/KWindowSystem/KXMessages
/usr/include/KF5/KWindowSystem/NETWM
/usr/include/KF5/KWindowSystem/config-kwindowsystem.h
/usr/include/KF5/KWindowSystem/fixx11h.h
/usr/include/KF5/KWindowSystem/kkeyserver.h
/usr/include/KF5/KWindowSystem/kkeyserver_x11.h
/usr/include/KF5/KWindowSystem/kmanagerselection.h
/usr/include/KF5/KWindowSystem/kselectionowner.h
/usr/include/KF5/KWindowSystem/kselectionwatcher.h
/usr/include/KF5/KWindowSystem/kstartupinfo.h
/usr/include/KF5/KWindowSystem/kusertimestamp.h
/usr/include/KF5/KWindowSystem/kwindoweffects.h
/usr/include/KF5/KWindowSystem/kwindowinfo.h
/usr/include/KF5/KWindowSystem/kwindowsystem.h
/usr/include/KF5/KWindowSystem/kwindowsystem_export.h
/usr/include/KF5/KWindowSystem/kxmessages.h
/usr/include/KF5/KWindowSystem/netwm.h
/usr/include/KF5/KWindowSystem/netwm_def.h
/usr/include/KF5/KWindowSystem/private/
/usr/include/KF5/KWindowSystem/private/kwindoweffects_p.h
/usr/include/KF5/KWindowSystem/private/kwindowinfo_p.h
/usr/include/KF5/KWindowSystem/private/kwindowsystem_p.h
/usr/include/KF5/KWindowSystem/private/kwindowsystemplugininterface_p.h