deps: allow crypto/x509 to use syscall

Allows the Windows part of CL 5700087 to land.

I had build tested CL 5753060 (which allows
crypto/x509 to use cgo and io/ioutil), and
didn't spot any errors on Windows.

Turns out I was wrong.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5753065

src/pkg/go/build/deps_test.go

@@ -300,7 +300,7 @@ var pkgDeps = map[string][]string{
 		"L4", "CRYPTO-MATH", "CGO", "OS",
 		"crypto/x509", "encoding/pem", "net", "syscall",
 	},
-	"crypto/x509":      {"L4", "CRYPTO-MATH", "OS", "CGO", "crypto/x509/pkix", "encoding/pem"},
+	"crypto/x509":      {"L4", "CRYPTO-MATH", "OS", "CGO", "crypto/x509/pkix", "encoding/pem", "syscall"},
 	"crypto/x509/pkix": {"L4", "CRYPTO-MATH"},
 
 	// Simple net+crypto-aware packages.