go.tools/ssa/interp: fix build breakage.
go.tools/ssa/interp: fix build breakage.

(intersecting changes in client)

R=gri, gri
CC=golang-dev
https://codereview.appspot.com/24100046

chenjianlong/go.tools,lattera/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,lattera/go.tools,lattera/go.tools,karmakaze/go.tools,chenjianlong/go.tools