/usr/include/claws-mail/gtk/ is in libclaws-mail-dev 3.16.0-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/claws-mail/gtk/about.h
/usr/include/claws-mail/gtk/authors.h
/usr/include/claws-mail/gtk/claws-marshal.h
/usr/include/claws-mail/gtk/colorlabel.h
/usr/include/claws-mail/gtk/colorsel.h
/usr/include/claws-mail/gtk/combobox.h
/usr/include/claws-mail/gtk/description_window.h
/usr/include/claws-mail/gtk/filesel.h
/usr/include/claws-mail/gtk/foldersort.h
/usr/include/claws-mail/gtk/gdkkeysyms-new.h
/usr/include/claws-mail/gtk/gtkaspell.h
/usr/include/claws-mail/gtk/gtkcmclist.h
/usr/include/claws-mail/gtk/gtkcmctree.h
/usr/include/claws-mail/gtk/gtkcmoptionmenu.h
/usr/include/claws-mail/gtk/gtksctree.h
/usr/include/claws-mail/gtk/gtkshruler.h
/usr/include/claws-mail/gtk/gtkunit.h
/usr/include/claws-mail/gtk/gtkutils.h
/usr/include/claws-mail/gtk/gtkvscrollbutton.h
/usr/include/claws-mail/gtk/headers.h
/usr/include/claws-mail/gtk/icon_legend.h
/usr/include/claws-mail/gtk/inputdialog.h
/usr/include/claws-mail/gtk/logwindow.h
/usr/include/claws-mail/gtk/manage_window.h
/usr/include/claws-mail/gtk/menu.h
/usr/include/claws-mail/gtk/pluginwindow.h
/usr/include/claws-mail/gtk/prefswindow.h
/usr/include/claws-mail/gtk/progressdialog.h
/usr/include/claws-mail/gtk/quicksearch.h
/usr/include/claws-mail/gtk/spell_entry.h
/usr/include/claws-mail/gtk/sslcertwindow.h