Make it possible to compile with gcc4.6
diff --git a/test/cpp/interop/server.cc b/test/cpp/interop/server.cc
index 263bd8e..9810ff6 100644
--- a/test/cpp/interop/server.cc
+++ b/test/cpp/interop/server.cc
@@ -36,6 +36,7 @@
 #include <thread>
 
 #include <signal.h>
+#include <unistd.h>
 
 #include <gflags/gflags.h>
 #include <grpc/grpc.h>
@@ -222,7 +223,7 @@
   std::unique_ptr<Server> server(builder.BuildAndStart());
   gpr_log(GPR_INFO, "Server listening on %s", server_address.str().c_str());
   while (!got_sigint) {
-    std::this_thread::sleep_for(std::chrono::seconds(5));
+    sleep(5);
   }
 }