/usr/include/KF5/messageviewer/ is in libkf5messageviewer-dev 4:17.12.3-0ubuntu3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/KF5/messageviewer/bodyparturlhandler.h
/usr/include/KF5/messageviewer/config-messageviewer.h
/usr/include/KF5/messageviewer/configurewidget.h
/usr/include/KF5/messageviewer/csshelper.h
/usr/include/KF5/messageviewer/csshelperbase.h
/usr/include/KF5/messageviewer/editorwatcher.h
/usr/include/KF5/messageviewer/globalsettings_messageviewer.h
/usr/include/KF5/messageviewer/grantleeheaderstyle.h
/usr/include/KF5/messageviewer/grantleeheaderteststyle.h
/usr/include/KF5/messageviewer/headerstrategy.h
/usr/include/KF5/messageviewer/headerstyle.h
/usr/include/KF5/messageviewer/headerstyle_util.h
/usr/include/KF5/messageviewer/headerstyleinterface.h
/usr/include/KF5/messageviewer/headerstylemenumanager.h
/usr/include/KF5/messageviewer/headerstyleplugin.h
/usr/include/KF5/messageviewer/headerstylepluginmanager.h
/usr/include/KF5/messageviewer/htmlblock.h
/usr/include/KF5/messageviewer/iconnamecache.h
/usr/include/KF5/messageviewer/invitationsettings.h
/usr/include/KF5/messageviewer/kxface.h
/usr/include/KF5/messageviewer/mailwebenginepage.h
/usr/include/KF5/messageviewer/mailwebengineview.h
/usr/include/KF5/messageviewer/markmessagereadhandler.h
/usr/include/KF5/messageviewer/messagepartrendererbase.h
/usr/include/KF5/messageviewer/messagepartrenderermanager.h
/usr/include/KF5/messageviewer/messagepartrenderplugin.h
/usr/include/KF5/messageviewer/messageviewer_debug.h
/usr/include/KF5/messageviewer/messageviewer_export.h
/usr/include/KF5/messageviewer/messageviewersettings.h
/usr/include/KF5/messageviewer/messageviewerutil.h
/usr/include/KF5/messageviewer/mimetype.h
/usr/include/KF5/messageviewer/objecttreeemptysource.h
/usr/include/KF5/messageviewer/plainheaderstyle.h
/usr/include/KF5/messageviewer/printingsettings.h
/usr/include/KF5/messageviewer/richheaderstrategy.h
/usr/include/KF5/messageviewer/scamcheckshorturl.h
/usr/include/KF5/messageviewer/scamcheckshorturlmanager.h
/usr/include/KF5/messageviewer/scamexpandurljob.h
/usr/include/KF5/messageviewer/spamheaderanalyzer.h
/usr/include/KF5/messageviewer/stl_util.h
/usr/include/KF5/messageviewer/urlhandler.h
/usr/include/KF5/messageviewer/viewer.h
/usr/include/KF5/messageviewer/viewerplugin.h
/usr/include/KF5/messageviewer/viewerplugininterface.h
/usr/include/KF5/messageviewer/viewerpluginmanager.h
/usr/include/KF5/messageviewer/viewerplugintoolmanager.h