Fix naming issue in stress_test

(via ahh@google.com)
Under some configurations gRPC includes google3 base headers.  google3
has a namespace "thread" which mixes poorly with unqualified use of the
type std::thread; adding that namespace to various google3 headers
breaks this test.

test/cpp/interop/stress_test.cc

@@ -202,7 +202,7 @@ int main(int argc, char** argv) {
 
   gpr_log(GPR_INFO, "Starting test(s)..");
 
-  vector<thread> test_threads;
+  vector<std::thread> test_threads;
   int thread_idx = 0;
   for (auto it = server_addresses.begin(); it != server_addresses.end(); it++) {
     StressTestInteropClient* client = new StressTestInteropClient(
@@ -210,7 +210,7 @@ int main(int argc, char** argv) {
         FLAGS_sleep_duration_ms);
 
     test_threads.emplace_back(
-        thread(&StressTestInteropClient::MainLoop, client));
+        std::thread(&StressTestInteropClient::MainLoop, client));
   }
 
   for (auto it = test_threads.begin(); it != test_threads.end(); it++) {