cmd/callgraph: fix build
cmd/callgraph: fix build

TBR=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/171640043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools