initialize static const outside of class

tools/db_stress.cc

@@ -521,7 +521,7 @@ class Stats {
 // State shared by all concurrent executions of the same benchmark.
 class SharedState {
  public:
-  static const uint32_t SENTINEL = 0xffffffff;
+  static const uint32_t SENTINEL;
 
   explicit SharedState(StressTest* stress_test) :
       cv_(&mu_),
@@ -676,6 +676,8 @@ class SharedState {
   std::vector<std::vector<port::Mutex>> key_locks_;
 };
 
+const uint32_t SharedState::SENTINEL = 0xffffffff;
+
 // Per-thread state for concurrent executions of the same benchmark.
 struct ThreadState {
   uint32_t tid; // 0..n-1