/usr/include/tcl8.6/tk-private/ is in tk8.6-dev 8.6.5-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/tcl8.6/tk-private/compat/
/usr/include/tcl8.6/tk-private/compat/stdlib.h
/usr/include/tcl8.6/tk-private/compat/unistd.h
/usr/include/tcl8.6/tk-private/generic/
/usr/include/tcl8.6/tk-private/generic/default.h
/usr/include/tcl8.6/tk-private/generic/ks_names.h
/usr/include/tcl8.6/tk-private/generic/tk.h
/usr/include/tcl8.6/tk-private/generic/tk3d.h
/usr/include/tcl8.6/tk-private/generic/tkBusy.h
/usr/include/tcl8.6/tk-private/generic/tkButton.h
/usr/include/tcl8.6/tk-private/generic/tkCanvas.h
/usr/include/tcl8.6/tk-private/generic/tkColor.h
/usr/include/tcl8.6/tk-private/generic/tkDecls.h
/usr/include/tcl8.6/tk-private/generic/tkEntry.h
/usr/include/tcl8.6/tk-private/generic/tkFileFilter.h
/usr/include/tcl8.6/tk-private/generic/tkFont.h
/usr/include/tcl8.6/tk-private/generic/tkImgPhoto.h
/usr/include/tcl8.6/tk-private/generic/tkInt.h
/usr/include/tcl8.6/tk-private/generic/tkIntDecls.h
/usr/include/tcl8.6/tk-private/generic/tkIntPlatDecls.h
/usr/include/tcl8.6/tk-private/generic/tkIntXlibDecls.h
/usr/include/tcl8.6/tk-private/generic/tkMenu.h
/usr/include/tcl8.6/tk-private/generic/tkMenubutton.h
/usr/include/tcl8.6/tk-private/generic/tkPlatDecls.h
/usr/include/tcl8.6/tk-private/generic/tkPort.h
/usr/include/tcl8.6/tk-private/generic/tkScale.h
/usr/include/tcl8.6/tk-private/generic/tkScrollbar.h
/usr/include/tcl8.6/tk-private/generic/tkSelect.h
/usr/include/tcl8.6/tk-private/generic/tkText.h
/usr/include/tcl8.6/tk-private/generic/tkUndo.h
/usr/include/tcl8.6/tk-private/generic/ttk/
/usr/include/tcl8.6/tk-private/generic/ttk/ttkDecls.h
/usr/include/tcl8.6/tk-private/generic/ttk/ttkManager.h
/usr/include/tcl8.6/tk-private/generic/ttk/ttkTheme.h
/usr/include/tcl8.6/tk-private/generic/ttk/ttkThemeInt.h
/usr/include/tcl8.6/tk-private/generic/ttk/ttkWidget.h
/usr/include/tcl8.6/tk-private/unix/
/usr/include/tcl8.6/tk-private/unix/tkUnixDefault.h
/usr/include/tcl8.6/tk-private/unix/tkUnixInt.h
/usr/include/tcl8.6/tk-private/unix/tkUnixPort.h