cgo: fix definition of opaque types
cgo: fix definition of opaque types

Fixes issue 3082.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5683074

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang