fix compile for VS2015

Summary:
Without the cast, the build will break on Windows.
Closes https://github.com/facebook/rocksdb/pull/1982

Differential Revision: D4690462

Pulled By: ajkr

fbshipit-source-id: c493b6c

util/histogram.cc

@@ -235,7 +235,7 @@ void HistogramStat::Data(HistogramData * const data) const {
   data->median = Median();
   data->percentile95 = Percentile(95);
   data->percentile99 = Percentile(99);
-  data->max = max();
+  data->max = static_cast<double>(max());
   data->average = Average();
   data->standard_deviation = StandardDeviation();
 }