doc: Unpublish global_zombie_object and wl_interface_equal

Both global_zombie_object and wl_interface_equal are private, yet were
part of public documentation despite not being part of the public API.

Move these two definitions to the top of an existing doxygen \cond block,
which removes them from the public documentation.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>