/usr/include/claws-mail/plugins/notification_plugin/ is in claws-mail-multi-notifier 3.8.0-1build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/eggaccelerators.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtk-hotkey-error.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtk-hotkey-info.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtk-hotkey-key-file-registry.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtk-hotkey-listener.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtk-hotkey-marshal.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtk-hotkey-registry.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtk-hotkey-utils.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtk-hotkey-x11-listener.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/gtkhotkey.h
/usr/include/claws-mail/plugins/notification_plugin/gtkhotkey/tomboykeybinder.h