Fix a compile error (#5864)

Summary:
```
tools/block_cache_analyzer/block_cache_trace_analyzer.cc:653:48: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'std::__1::linear_congruential_engine<unsigned int, 48271, 0, 2147483647>::result_type' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  std::default_random_engine rand_engine(env_->NowMicros());
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5864

Differential Revision: D17668962

fbshipit-source-id: e08fa58b2a78a8dd8b334862b5714208f696b8ab

tools/block_cache_analyzer/block_cache_trace_analyzer.cc

@@ -650,7 +650,7 @@ void BlockCacheTraceAnalyzer::WriteCorrelationFeaturesToFile(
     const std::map<std::string, Features>& label_features,
     const std::map<std::string, Predictions>& label_predictions,
     uint32_t max_number_of_values) const {
-  std::default_random_engine rand_engine(env_->NowMicros());
+  std::default_random_engine rand_engine(static_cast<unsigned int>(env_->NowMicros()));
   for (auto const& label_feature_vectors : label_features) {
     const Features& past = label_feature_vectors.second;
     auto it = label_predictions.find(label_feature_vectors.first);