Update 64-bit shift in compression.h

Summary:
This was failing the build on windows with zstd, warning treated as an error, 32-bit shift implicitly converted to 64-bit.
Closes https://github.com/facebook/rocksdb/pull/3624

Differential Revision: D7307883

Pulled By: gfosco

fbshipit-source-id: 68110e9b5b1b59b668dec6cf86b67556402574e7

util/compression.h

@@ -848,7 +848,7 @@ inline std::string ZSTD_TrainDictionary(const std::string& samples,
 #if ZSTD_VERSION_NUMBER >= 800  // v0.8.0+
   // skips potential partial sample at the end of "samples"
   size_t num_samples = samples.size() >> sample_len_shift;
-  std::vector<size_t> sample_lens(num_samples, 1 << sample_len_shift);
+  std::vector<size_t> sample_lens(num_samples, size_t(1) << sample_len_shift);
   return ZSTD_TrainDictionary(samples, sample_lens, max_dict_bytes);
 #else   // up to v0.7.x
   assert(false);