/usr/include/gtkextra-3.0/gtkextra/ is in libgtkextra-dev 3.3.3-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gtkextra-3.0/gtkextra/gtkbordercombo.h
/usr/include/gtkextra-3.0/gtkextra/gtkcharsel.h
/usr/include/gtkextra-3.0/gtkextra/gtkcheckitem.h
/usr/include/gtkextra-3.0/gtkextra/gtkcolorcombo.h
/usr/include/gtkextra-3.0/gtkextra/gtkcombobutton.h
/usr/include/gtkextra-3.0/gtkextra/gtkdataentry.h
/usr/include/gtkextra-3.0/gtkextra/gtkdataformat.h
/usr/include/gtkextra-3.0/gtkextra/gtkdatatextview.h
/usr/include/gtkextra-3.0/gtkextra/gtkdirtree.h
/usr/include/gtkextra-3.0/gtkextra/gtkextra-marshal.h
/usr/include/gtkextra-3.0/gtkextra/gtkextra.h
/usr/include/gtkextra-3.0/gtkextra/gtkextrafeatures.h
/usr/include/gtkextra-3.0/gtkextra/gtkextraicons.h
/usr/include/gtkextra-3.0/gtkextra/gtkextratypebuiltins.h
/usr/include/gtkextra-3.0/gtkextra/gtkfileicons.h
/usr/include/gtkextra-3.0/gtkextra/gtkfilelist.h
/usr/include/gtkextra-3.0/gtkextra/gtkfontcombo.h
/usr/include/gtkextra-3.0/gtkextra/gtkiconfilesel.h
/usr/include/gtkextra-3.0/gtkextra/gtkiconlist.h
/usr/include/gtkextra-3.0/gtkextra/gtkitementry.h
/usr/include/gtkextra-3.0/gtkextra/gtkplot.h
/usr/include/gtkextra-3.0/gtkextra/gtkplot3d.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotarray.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotbar.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotbox.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotbubble.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcairo.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcandle.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcanvas.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcanvasellipse.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcanvasline.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcanvaspixmap.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcanvasplot.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcanvasrectangle.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcanvastext.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotcsurface.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotdata.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotdt.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotflux.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotgdk.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotpc.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotpixmap.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotpolar.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotprint.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotps.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotsegment.h
/usr/include/gtkextra-3.0/gtkextra/gtkplotsurface.h
/usr/include/gtkextra-3.0/gtkextra/gtkpsfont.h
/usr/include/gtkextra-3.0/gtkextra/gtksheet.h
/usr/include/gtkextra-3.0/gtkextra/gtksheetcolumn.h
/usr/include/gtkextra-3.0/gtkextra/gtktogglecombo.h