Fix release warning for unused bg_canceled

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6357

Differential Revision: D19670931

Pulled By: maysamyabandeh

fbshipit-source-id: d528c4c7f9450f1f38b9d2a36e0d5d0865b39be9

db_stress_tool/db_stress_test_base.cc

@@ -2085,6 +2085,7 @@ void StressTest::Reopen(ThreadState* thread) {
     bg_canceled = wait;
   }
   assert(!write_prepared || bg_canceled);
+  (void) bg_canceled;
 #else
   (void) thread;
 #endif