build fix

Author: DmitriyMusatkin

tests/s3_data_plane_tests.c

@@ -2325,7 +2325,7 @@ static int s_test_s3_put_object_buffer_pool_trim(struct aws_allocator *allocator
     ASSERT_SUCCESS(aws_s3_tester_send_meta_request_with_options(&tester, &put_options, NULL));
 
     struct aws_s3_default_buffer_pool_usage_stats usage_before =
-        aws_s3_default_buffer_pool_get_usage((struct aws_s3_default_buffer_pool *)client->buffer_pool->impl);
+        aws_s3_default_buffer_pool_get_usage(client->buffer_pool);
 
     ASSERT_TRUE(0 != usage_before.primary_num_blocks);