/usr/include/gimp-2.0/libgimpwidgets/ is in libgimp2.0-dev 2.8.14-1+deb8u2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gimp-2.0/libgimpwidgets/gimp3migration.h
/usr/include/gimp-2.0/libgimpwidgets/gimpbrowser.h
/usr/include/gimp-2.0/libgimpwidgets/gimpbutton.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcairo-utils.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcellrenderercolor.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcellrenderertoggle.h
/usr/include/gimp-2.0/libgimpwidgets/gimpchainbutton.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorarea.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorbutton.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolordisplay.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolordisplaystack.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorhexentry.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolornotebook.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorprofilecombobox.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorprofilestore.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorscale.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorscales.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorselect.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorselection.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcolorselector.h
/usr/include/gimp-2.0/libgimpwidgets/gimpcontroller.h
/usr/include/gimp-2.0/libgimpwidgets/gimpdialog.h
/usr/include/gimp-2.0/libgimpwidgets/gimpenumcombobox.h
/usr/include/gimp-2.0/libgimpwidgets/gimpenumlabel.h
/usr/include/gimp-2.0/libgimpwidgets/gimpenumstore.h
/usr/include/gimp-2.0/libgimpwidgets/gimpenumwidgets.h
/usr/include/gimp-2.0/libgimpwidgets/gimpfileentry.h
/usr/include/gimp-2.0/libgimpwidgets/gimpframe.h
/usr/include/gimp-2.0/libgimpwidgets/gimphelpui.h
/usr/include/gimp-2.0/libgimpwidgets/gimphintbox.h
/usr/include/gimp-2.0/libgimpwidgets/gimpintcombobox.h
/usr/include/gimp-2.0/libgimpwidgets/gimpintstore.h
/usr/include/gimp-2.0/libgimpwidgets/gimpmemsizeentry.h
/usr/include/gimp-2.0/libgimpwidgets/gimpnumberpairentry.h
/usr/include/gimp-2.0/libgimpwidgets/gimpoffsetarea.h
/usr/include/gimp-2.0/libgimpwidgets/gimpoldwidgets.h
/usr/include/gimp-2.0/libgimpwidgets/gimppageselector.h
/usr/include/gimp-2.0/libgimpwidgets/gimppatheditor.h
/usr/include/gimp-2.0/libgimpwidgets/gimppickbutton.h
/usr/include/gimp-2.0/libgimpwidgets/gimppixmap.h
/usr/include/gimp-2.0/libgimpwidgets/gimppreview.h
/usr/include/gimp-2.0/libgimpwidgets/gimppreviewarea.h
/usr/include/gimp-2.0/libgimpwidgets/gimppropwidgets.h
/usr/include/gimp-2.0/libgimpwidgets/gimpquerybox.h
/usr/include/gimp-2.0/libgimpwidgets/gimpruler.h
/usr/include/gimp-2.0/libgimpwidgets/gimpscaleentry.h
/usr/include/gimp-2.0/libgimpwidgets/gimpscrolledpreview.h
/usr/include/gimp-2.0/libgimpwidgets/gimpsizeentry.h
/usr/include/gimp-2.0/libgimpwidgets/gimpstock.h
/usr/include/gimp-2.0/libgimpwidgets/gimpstringcombobox.h
/usr/include/gimp-2.0/libgimpwidgets/gimpunitcombobox.h
/usr/include/gimp-2.0/libgimpwidgets/gimpunitmenu.h
/usr/include/gimp-2.0/libgimpwidgets/gimpunitstore.h
/usr/include/gimp-2.0/libgimpwidgets/gimpwidgets-error.h
/usr/include/gimp-2.0/libgimpwidgets/gimpwidgets.h
/usr/include/gimp-2.0/libgimpwidgets/gimpwidgetsenums.h
/usr/include/gimp-2.0/libgimpwidgets/gimpwidgetstypes.h
/usr/include/gimp-2.0/libgimpwidgets/gimpzoommodel.h