veyron/security/agent: Fix the build. Pass passphrase param of nil.

Change-Id: Ie79c6149e356cef95be4524834dfc9f735c10eb1
diff --git a/runtimes/google/rt/security.go b/runtimes/google/rt/security.go
index e402cee..5e6ea1b 100644
--- a/runtimes/google/rt/security.go
+++ b/runtimes/google/rt/security.go
@@ -225,7 +225,7 @@
 	keyPath := path.Join(dir, privateKeyFile)
 	if f, err := os.Open(keyPath); err == nil {
 		defer f.Close()
-		v, err := vsecurity.LoadPEMKey(f)
+		v, err := vsecurity.LoadPEMKey(f, nil)
 		if err != nil {
 			return nil, err
 		}
@@ -248,7 +248,7 @@
 		return nil, err
 	}
 	defer f.Close()
-	return key, vsecurity.SavePEMKey(f, key)
+	return key, vsecurity.SavePEMKey(f, key, nil)
 }
 
 func initStoreAndRootsFromCredentials(dir string, secsigner security.Signer) (security.BlessingStore, security.BlessingRoots, error) {