Fix portability problem.

src/core/lib/channel/message_size_filter.c

@@ -67,7 +67,7 @@ static void recv_message_ready(grpc_exec_ctx* exec_ctx, void* user_data,
   if ((*calld->recv_message)->length > chand->max_recv_size) {
     char* message_string;
     gpr_asprintf(&message_string, "Received message larger than max (%lu)",
-                 chand->max_recv_size);
+                 (unsigned long)chand->max_recv_size);
     gpr_slice message = gpr_slice_from_copied_string(message_string);
     gpr_free(message_string);
     grpc_call_element_send_cancel_with_message(
@@ -89,7 +89,7 @@ static void start_transport_stream_op(grpc_exec_ctx* exec_ctx,
       op->send_message->length > chand->max_send_size) {
     char* message_string;
     gpr_asprintf(&message_string, "Sent message larger than max (%lu)",
-                 chand->max_send_size);
+                 (unsigned long)chand->max_send_size);
     gpr_slice message = gpr_slice_from_copied_string(message_string);
     gpr_free(message_string);
     grpc_call_element_send_cancel_with_message(