cmd/vet: use changed types.LookupParent API (fix build)
cmd/vet: use changed types.LookupParent API (fix build)

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/162080043

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