Fix build when stat buckets or cost-benefit-stats are not enabled

Summary: memcached fails to build if not configured with --enable-stat-buckets   and  --enable-cost-benefit-stats

Reviewed By: marc

Test Plan: it builds.. no real functional changes

Revert: OK

src/stats.c

@@ -45,13 +45,17 @@ static int num_prefixes = 0;
 static int total_prefix_size = 0;
 static PREFIX_STATS wildcard;
 
+#if defined(STATS_BUCKETS)
 SIZE_BUCKETS set;
 SIZE_BUCKETS get;
 SIZE_BUCKETS evict;
 SIZE_BUCKETS delete;
 SIZE_BUCKETS overwrite;
+#endif
 
+#if defined(COST_BENEFIT_STATS)
 cost_benefit_buckets_t cb_buckets;
+#endif
 
 void stats_prefix_init() {
     memset(prefix_stats, 0, sizeof(prefix_stats));