syscall: add missing SO_ERROR constant for nacl/*

CL 69340044 requires that syscall.SO_ERROR be defined on all unix like platforms. Add SO_ERROR to the list of dummy constants in sycall/net_nacl.go.

LGTM=bradfitz
R=iant, rsc, mikioh.mikioh, bradfitz
CC=golang-codereviews
https://golang.org/cl/73100043

src/pkg/syscall/net_nacl.go

@@ -112,6 +112,7 @@ const (
 	SO_SNDBUF
 	SO_KEEPALIVE
 	SO_LINGER
+	SO_ERROR
 	IP_MULTICAST_IF
 	IP_MULTICAST_LOOP
 	IP_ADD_MEMBERSHIP