Fix build

Summary: Build broken by https://github.com/facebook/rocksdb/commit/6ede020dc419c1621254f26060076ee6d2c2d792

Test Plan: make all

Reviewers: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D37689

tools/db_stress.cc

@@ -1063,7 +1063,7 @@ class StressTest {
       }
 
       auto thread_pool_size_base = FLAGS_max_background_compactions;
-      auto thread_pool_size_var = FLAGS_compaction_thread_pool_varations;
+      auto thread_pool_size_var = FLAGS_compaction_thread_pool_variations;
       int new_thread_pool_size =
           thread_pool_size_base - thread_pool_size_var +
           thread->rand.Next() % (thread_pool_size_var * 2 + 1);