graphics/graphene: Unbreak build with GTKDOC=on

pkg-static: Unable to access file [...]share/gtk-doc/html/graphene/object-tree.html:No such file or directory

PR: 232762
Reported by: Hyun Hwang <hyun@caffeinated.codes>
Submitted by: Ting-Wei Lan <lantw44@gmail.com>
Approved by: gnome (maintainer timeout, ~2 months)