fix compilation error: implicit conversion loses integer precision

Summary:
Fix compilation error with clang:
> tools/db_stress.cc:2598:21: error: implicit conversion loses integer precision: 'gflags::uint64' (aka 'unsigned long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
        Random rand(FLAGS_seed);
               ~~~~ ^~~~~~~~~~
Closes https://github.com/facebook/rocksdb/pull/3746

Differential Revision: D7703209

Pulled By: miasantreble

fbshipit-source-id: 18c56a5138a2f308e4213594bc82e8e64bc21570

tools/db_stress.cc

@@ -2595,7 +2595,7 @@ class StressTest {
         // after a crash, rollback to commit recovered transactions
         std::vector<Transaction*> trans;
         txn_db_->GetAllPreparedTransactions(&trans);
-        Random rand(FLAGS_seed);
+        Random rand(static_cast<uint32_t>(FLAGS_seed));
         for (auto txn : trans) {
           if (rand.OneIn(2)) {
             s = txn->Commit();