Fixes userspace compile error:

drm/sis_drm.h:68:19: error: field ‘obj_list’ has incomplete type
  struct list_head obj_list;

Suggested by Emil Velikov <emil.l.velikov@gmail.com> at
https://lkml.org/lkml/2015/6/3/792Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Dave Airlie <airlied@redhat.com>