slqb: fix compilation warning

gather_stats is not used if CONFIG_SLQB_SYSFS is not selected. Make
it conditional and avoid the warning.
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slqb.c

@@ -3121,6 +3121,7 @@ static void gather_stats_locked(struct kmem_cache *s,
 	stats->nr_objects = stats->nr_slabs * s->objects;
 }
 
+#ifdef CONFIG_SLQB_SYSFS
 static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
 {
 	down_read(&slqb_lock); /* hold off hotplug */
@@ -3128,6 +3129,7 @@ static void gather_stats(struct kmem_cache *s, struct stats_gather *stats)
 	up_read(&slqb_lock);
 }
 #endif
+#endif
 
 /*
  * The /proc/slabinfo ABI