Add missing use-statement

Author: tvdijen

src/XML/SignableElementTrait.php

@@ -6,6 +6,7 @@
 
 use DOMElement;
 use SimpleSAML\Assert\Assert;
+use SimpleSAML\SAML2\Compat\ContainerSingleton;
 use SimpleSAML\XML\DOMDocumentFactory;
 use SimpleSAML\XMLSecurity\Alg\Signature\SignatureAlgorithmInterface;
 use SimpleSAML\XMLSecurity\Constants as C;