encoder/filename: Wrap scsu package

lib/encoder/filename/scsu.go

@@ -0,0 +1,16 @@
+package filename
+
+import "errors"
+
+const scsuNotEnabled = "scsu encoding not enabled in this build due to old go version"
+
+// Functions wrap scsu package, since it doesn't build on old Go versions.
+// Remove once v1.13 is minimum supported version.
+
+var scsuDecode = func(b []byte) (string, error) {
+	return "", errors.New(scsuNotEnabled)
+}
+
+var scsuEncodeStrict = func(src string, dst []byte) ([]byte, error) {
+	return nil, errors.New(scsuNotEnabled)
+}