go.tools/dashboard/builder: fix imported by not used error
go.tools/dashboard/builder: fix imported by not used error

Update issue 7372

This slipped into the build in revision 4b4229fc616b.

Thanks to @pierredurand87 for noticing the break.

LGTM=bgarcia
R=golang-codereviews, bgarcia
CC=golang-codereviews
https://codereview.appspot.com/66670043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old