slub: Put objects_show() into CONFIG_SLUB_DEBUG enabled block

The SO_ALL|SO_OBJECTS pair is only used when enabling CONFIG_SLUB_DEBUG
option, so the objects_show() definition should be surrounded by
CONFIG_SLUB_DEBUG too.

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>