SLUB: Fix memory hotplug with !NUMA

This patch fixes the following build breakage when memory hotplug is enabled on
UMA configurations:

  /home/test/linux-2.6/mm/slub.c: In function 'kmem_cache_init':
  /home/test/linux-2.6/mm/slub.c:3031:2: error: 'slab_memory_callback'
  undeclared (first use in this function)
  /home/test/linux-2.6/mm/slub.c:3031:2: note: each undeclared
  identifier is reported only once for each function it appears in
  make[2]: *** [mm/slub.o] Error 1
  make[1]: *** [mm] Error 2
  make: *** [sub-make] Error 2

Reported-by: Zimny Lech <napohybelskurwysynom2010@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -2851,7 +2851,7 @@ int kmem_cache_shrink(struct kmem_cache *s)
 }
 EXPORT_SYMBOL(kmem_cache_shrink);
 
-#if defined(CONFIG_NUMA) && defined(CONFIG_MEMORY_HOTPLUG)
+#if defined(CONFIG_MEMORY_HOTPLUG)
 static int slab_mem_going_offline_callback(void *arg)
 {
 	struct kmem_cache *s;