/usr/include/KF5/MessageViewer/ is in libkf5messageviewer-dev 4:16.04.3-3~deb9u1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/KF5/MessageViewer/AdBlockBlockableItemsDialog
/usr/include/KF5/MessageViewer/AdBlockListWidget
/usr/include/KF5/MessageViewer/AdBlockManager
/usr/include/KF5/MessageViewer/AdBlockSettingWidget
/usr/include/KF5/MessageViewer/AttachmentStrategy
/usr/include/KF5/MessageViewer/BodyPart
/usr/include/KF5/MessageViewer/BodyPartFormatter
/usr/include/KF5/MessageViewer/BodyPartFormatterBaseFactory
/usr/include/KF5/MessageViewer/BodyPartFormatterFactory
/usr/include/KF5/MessageViewer/BodyPartURLHandler
/usr/include/KF5/MessageViewer/CSSHelper
/usr/include/KF5/MessageViewer/CSSHelperBase
/usr/include/KF5/MessageViewer/ConfigureWidget
/usr/include/KF5/MessageViewer/CreateEventJob
/usr/include/KF5/MessageViewer/CreateNoteJob
/usr/include/KF5/MessageViewer/CreateTodoJob
/usr/include/KF5/MessageViewer/EditorWatcher
/usr/include/KF5/MessageViewer/Enums
/usr/include/KF5/MessageViewer/FileHtmlWriter
/usr/include/KF5/MessageViewer/FindBarBase
/usr/include/KF5/MessageViewer/FindBarWebView
/usr/include/KF5/MessageViewer/GrantleeHeaderStyle
/usr/include/KF5/MessageViewer/GrantleeHeaderTestStyle
/usr/include/KF5/MessageViewer/HeaderStrategy
/usr/include/KF5/MessageViewer/HeaderStyle
/usr/include/KF5/MessageViewer/HeaderStyleInterface
/usr/include/KF5/MessageViewer/HeaderStyleMenuManager
/usr/include/KF5/MessageViewer/HeaderStylePlugin
/usr/include/KF5/MessageViewer/HeaderStylePluginManager
/usr/include/KF5/MessageViewer/HeaderStyle_Util
/usr/include/KF5/MessageViewer/HtmlWriter
/usr/include/KF5/MessageViewer/IconNameCache
/usr/include/KF5/MessageViewer/InvitationSettings
/usr/include/KF5/MessageViewer/KXFace
/usr/include/KF5/MessageViewer/MailWebView
/usr/include/KF5/MessageViewer/MarkMessageReadHandler
/usr/include/KF5/MessageViewer/MessagePart
/usr/include/KF5/MessageViewer/MessageViewerSettings
/usr/include/KF5/MessageViewer/MessageViewerUtil
/usr/include/KF5/MessageViewer/MimeType
/usr/include/KF5/MessageViewer/NodeHelper
/usr/include/KF5/MessageViewer/ObjectTreeEmptySource
/usr/include/KF5/MessageViewer/ObjectTreeParser
/usr/include/KF5/MessageViewer/ObjectTreeSourceIf
/usr/include/KF5/MessageViewer/PartMetaData
/usr/include/KF5/MessageViewer/PlainHeaderStyle
/usr/include/KF5/MessageViewer/PrintingSettings
/usr/include/KF5/MessageViewer/RichHeaderStrategy
/usr/include/KF5/MessageViewer/ScamCheckShortUrl
/usr/include/KF5/MessageViewer/ScamCheckShortUrlManager
/usr/include/KF5/MessageViewer/ScamExpandUrlJob
/usr/include/KF5/MessageViewer/SpamHeaderAnalyzer
/usr/include/KF5/MessageViewer/Stl_Util
/usr/include/KF5/MessageViewer/URLHandler
/usr/include/KF5/MessageViewer/Viewer
/usr/include/KF5/MessageViewer/ViewerPlugin
/usr/include/KF5/MessageViewer/ViewerPluginInterface
/usr/include/KF5/MessageViewer/ViewerPluginManager
/usr/include/KF5/MessageViewer/ViewerPluginToolManager
/usr/include/KF5/MessageViewer/WebKitPartHtmlWriter
/usr/include/KF5/MessageViewer/WebPage
/usr/include/KF5/MessageViewer/WebViewAccessKey
/usr/include/KF5/MessageViewer/ZoomActionMenu