Add include guard

Author: kingcrimsontianyu

cpp/include/kvikio/function_wrapper.hpp

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#pragma once
+
 #include <functional>
 #include <memory>
 

cpp/include/kvikio/threadpool_simple.hpp

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#pragma once
+
 #include <atomic>
 #include <condition_variable>
 #include <future>