Fixing comile errors on OS X

Summary: Building master on OS X has some compile errors due to implicit type conversions which generate warnings which RocksDB's build settings raise as errors.

Test Plan: It compiles!

Reviewers: ljin, igor

Reviewed By: ljin

Differential Revision: https://reviews.facebook.net/D24135

db/db_bench.cc

@@ -1278,7 +1278,8 @@ class Benchmark {
         method = &Benchmark::ReadRandomFast;
       } else if (name == Slice("multireadrandom")) {
         entries_per_batch_ = FLAGS_batch_size;
-        fprintf(stderr, "entries_per_batch_ = %ld\n", entries_per_batch_);
+        fprintf(stderr, "entries_per_batch = %" PRIi64 "\n",
+                entries_per_batch_);
         method = &Benchmark::MultiReadRandom;
       } else if (name == Slice("readmissing")) {
         ++key_size_;