Fix lambda capture expression for windows

Summary:
there was an error when accessing kItersPerThread in the lambda:
https://ci.appveyor.com/project/Facebook/rocksdb/build/1.0.1654

Test Plan: doitlive

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D62379

util/thread_local_test.cc

@@ -466,7 +466,7 @@ TEST_F(ThreadLocalTest, Fold) {
   port::Mutex mu;
   port::CondVar cv(&mu);
   Params params(&mu, &cv, nullptr, kNumThreads, unref);
-  auto func = [](void* ptr) {
+  auto func = [&](void* ptr) {
     auto& p = *static_cast<Params*>(ptr);
     ASSERT_TRUE(p.tls1.Get() == nullptr);
     p.tls1.Reset(new std::atomic<int64_t>(0));