fix compile error with graphlab-pagerank
fix compile error with graphlab-pagerank

uwsampa/grappa,alexfrolov/grappa,alexfrolov/grappa,uwsampa/grappa,uwsampa/grappa,uwsampa/grappa,uwsampa/grappa,alexfrolov/grappa,uwsampa/grappa,alexfrolov/grappa,alexfrolov/grappa,uwsampa/grappa,alexfrolov/grappa,alexfrolov/grappa