Fix appveyor compliant about passing const to thread (#5447)

Summary:
CLANG would complain if we pass const to lambda function and appveyor complains if we don't (https://github.com/facebook/rocksdb/pull/5443). The patch fixes that by using the default capture mode.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5447

Differential Revision: D15788722

Pulled By: maysamyabandeh

fbshipit-source-id: 47e7f49264afe31fdafe42cb8bf93da126abfca9

utilities/transactions/write_prepared_transaction_test.cc

@@ -164,7 +164,7 @@ TEST(PreparedHeap, Concurrent) {
 
   for (size_t n = 0; n < 100; n++) {
     last = 0;
-    t[0] = rocksdb::port::Thread([&heap, &last]() {
+    t[0] = rocksdb::port::Thread([&]() {
       Random rnd(1103);
       for (size_t seq = 1; seq <= t_cnt; seq++) {
         // This is not recommended usage but we should be resilient against it.