go.crypto/curve25519: fix incorrect build tag
go.crypto/curve25519: fix incorrect build tag

The build tag was incorrect. CL 6458091 exposed the mistake.

R=golang-dev, dsymonds, r
CC=golang-dev
http://codereview.appspot.com/6460067

cmars/go.crypto,drhax9908/go.crypto,dizzyd/go.crypto,isislovecruft/go.crypto,polydawn-ports/go.crypto,xman1101/terminal,xman1101/terminal,isislovecruft/go.crypto,tonnerre/golang-go.crypto,cmars/go.crypto,cmars/sshlurp-go.crypto,dizzyd/go.crypto,drhax9908/go.crypto,tonnerre/golang-go.crypto,polydawn-ports/go.crypto,cmars/sshlurp-go.crypto