fix db_stress uint64_t to int32 cast

Summary:
Clang complain about an cast from uint64_t to int32 in db_stress. Fixing it.
Closes facebook#2755

Differential Revision: D5655947

Pulled By: yiwu-arbug

fbshipit-source-id: cfac10e796e0adfef4727090b50975b0d6e2c9be

tools/db_stress.cc

@@ -328,7 +328,7 @@ extern std::vector<std::string> rocksdb_kill_prefix_blacklist;
 
 DEFINE_bool(disable_wal, false, "If true, do not write WAL for write.");
 
-DEFINE_int32(target_file_size_base, rocksdb::Options().target_file_size_base,
+DEFINE_int64(target_file_size_base, rocksdb::Options().target_file_size_base,
              "Target level-1 file size for compaction");
 
 DEFINE_int32(target_file_size_multiplier, 1,