Add graphviz package dependency for xhprof

Xhprof has the ability to draw callgraphs, exposed in its interface
as the `View Callgraph` button.  Currently this just outputs an error
about a missing program, adding graphviz installs the appropriate
programs and restores this functionality.

Change-Id: If2e16dd6c71bf17e64997d06cc56033999f3b8da