HDFS-13722. HDFS Native Client Fails Compilation on Ubuntu 18.04 (contributed by Jack Bearden)

hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfspp/lib/rpc/request.cc

@@ -16,7 +16,7 @@
  * limitations under the License.
  */
 
-
+#include <functional>
 #include "request.h"
 #include "rpc_engine.h"
 #include "sasl_protocol.h"