Commit a76b28bd moved the dbus-interfaces to the
dbus-interfaces directory without changing the
path for docs build as well, resulting in a fail
of the build when building with gtk_doc=true.
So change the path in the build accordingly to
ensure that the build does not fail when building
with gtk_doc=true.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/236
4a4fa8b2d7