Fix build of GPU debugging code (#2071)

src/treelearner/gpu_tree_learner.cpp

@@ -436,7 +436,7 @@ void GPUTreeLearner::AllocateGPUMemory() {
     queue_.enqueue_write_buffer(device_features_->get_buffer(),
                         i * num_data_ * sizeof(Feature4), num_data_ * sizeof(Feature4), host4);
     #if GPU_DEBUG >= 1
-    printf("first example of feature-group tuple is: %d %d %d %d\n", host4[0].s0, host4[0].s1, host4[0].s2, host4[0].s3);
+    printf("first example of feature-group tuple is: %d %d %d %d\n", host4[0].s[0], host4[0].s[1], host4[0].s[2], host4[0].s[3]);
     printf("Feature-groups copied to device with multipliers ");
     for (int l = 0; l < dword_features_; ++l) {
       printf("%d ", dev_bin_mult[l]);