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

webfd/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,8limbs/gotools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools