Add a missing import so that the code compiles again

Author: benmccann

src/main/java/com/onelogin/saml/Response.java

@@ -25,6 +25,7 @@
 import javax.xml.xpath.XPathFactory;
 
 import org.apache.commons.codec.binary.Base64;
+import org.w3c.dom.Attr;
 import org.w3c.dom.Document;
 import org.w3c.dom.Element;
 import org.w3c.dom.NamedNodeMap;