Fix compile error on ubuntu
Fix compile error on ubuntu

yvxiang/dfs,myawan/bfs-1,bluebore/dfs,00k/ToyDFS,myawan/bfs-1,bluebore/bfs,linyvxiang/dfs,yvxiang/coverage_for_bfs,linyvxiang/dfs,yvxiang/coverage_for_bfs,baidu/bfs,imotai/bfs,00k/ToyDFS,lylei/dfs,imotai/bfs,myawan/bfs-1,lylei/dfs,baidu/bfs,yvxiang/dfs,yvxiang/dfs,yvxiang/coverage_for_bfs,linyvxiang/dfs,bluebore/bfs,bluebore/dfs,baidu/bfs,lylei/dfs,myawan/bfs-1,bluebore/dfs,baidu/bfs,yvxiang/dfs,imotai/bfs,yvxiang/coverage_for_bfs,bluebore/bfs,lylei/dfs