[Graph]: fix a warning in Ubuntu 14.04.
[Graph]: fix a warning in Ubuntu 14.04.

flashxio/FlashX,silky/FlashGraph,icoming/FlashX,icoming/FlashGraph,zheng-da/FlashX,silky/FlashGraph,icoming/FlashX,flashxio/FlashX,silky/FlashGraph,flashxio/FlashX,icoming/FlashGraph,icoming/FlashGraph,flashxio/FlashX,icoming/FlashGraph,silky/FlashGraph,zheng-da/FlashX,zheng-da/FlashX,silky/FlashGraph,zheng-da/FlashX,zheng-da/FlashX,icoming/FlashX,icoming/FlashX,icoming/FlashX,icoming/FlashGraph,flashxio/FlashX,flashxio/FlashX