/usr/include/KDecoration2/kdecoration2/ is in libkdecorations2-dev 4:5.8.4-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/KDecoration2/kdecoration2/decoratedclient.h
/usr/include/KDecoration2/kdecoration2/decoration.h
/usr/include/KDecoration2/kdecoration2/decorationbutton.h
/usr/include/KDecoration2/kdecoration2/decorationbuttongroup.h
/usr/include/KDecoration2/kdecoration2/decorationdefines.h
/usr/include/KDecoration2/kdecoration2/decorationsettings.h
/usr/include/KDecoration2/kdecoration2/decorationshadow.h
/usr/include/KDecoration2/kdecoration2/kdecoration2_export.h
/usr/include/KDecoration2/kdecoration2/private/
/usr/include/KDecoration2/kdecoration2/private/decoratedclientprivate.h
/usr/include/KDecoration2/kdecoration2/private/decorationbridge.h
/usr/include/KDecoration2/kdecoration2/private/decorationsettingsprivate.h
/usr/include/KDecoration2/kdecoration2/private/kdecoration2_private_export.h