/usr/include/claws-mail/plugins/ is in libclaws-mail-dev 3.11.1-3+deb8u1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/claws-mail/plugins/claws-mail/
/usr/include/claws-mail/plugins/claws-mail/archiver.h
/usr/include/claws-mail/plugins/claws-mail/archiver_prefs.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/eggaccelerators.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtk-hotkey-error.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtk-hotkey-info.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtk-hotkey-key-file-registry.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtk-hotkey-listener.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtk-hotkey-marshal.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtk-hotkey-registry.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtk-hotkey-utils.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtk-hotkey-x11-listener.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/gtkhotkey.h
/usr/include/claws-mail/plugins/claws-mail/gtkhotkey/tomboykeybinder.h
/usr/include/claws-mail/plugins/claws-mail/ical.h
/usr/include/claws-mail/plugins/pgpcore/
/usr/include/claws-mail/plugins/pgpcore/autocompletion.h
/usr/include/claws-mail/plugins/pgpcore/passphrase.h
/usr/include/claws-mail/plugins/pgpcore/pgp_utils.h
/usr/include/claws-mail/plugins/pgpcore/pgp_viewer.h
/usr/include/claws-mail/plugins/pgpcore/prefs_gpg.h
/usr/include/claws-mail/plugins/pgpcore/select-keys.h
/usr/include/claws-mail/plugins/pgpcore/sgpgme.h
/usr/include/claws-mail/plugins/pgpinline/
/usr/include/claws-mail/plugins/pgpinline/pgpinline.h
/usr/include/claws-mail/plugins/pgpmime/
/usr/include/claws-mail/plugins/pgpmime/pgpmime.h
/usr/include/claws-mail/plugins/smime/
/usr/include/claws-mail/plugins/smime/smime.h
/usr/include/claws-mail/plugins/vcalendar/
/usr/include/claws-mail/plugins/vcalendar/vcal_interface.h