cmd/cgo: silence const warnings
cmd/cgo: silence const warnings

Fixes issue 3152.

R=golang-dev, dsymonds, r
CC=golang-dev
http://codereview.appspot.com/5786047

dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang