IssueXML.php: skip DOCTYPE (fixes cobertura)

skips '<!DOCTYPE coverage SYSTEM ...' (cobertura) and uses the following element instead

src/PHPCodeBrowser/IssueXML.php

@@ -56,6 +56,7 @@
 namespace PHPCodeBrowser;
 
 use DOMDocument;
+use DOMDocumentType;
 use DOMNode;
 use DOMNodeList;
 use DOMXPath;
@@ -155,6 +156,10 @@ public function addDirectory(string $directory): IssueXML
     public function addXMLFile(DOMDocument $domDocument): void
     {
         foreach ($domDocument->childNodes as $node) {
+            if ($node instanceof DOMDocumentType) {
+                continue;
+            }
+
             $this->documentElement->appendChild($this->importNode($node, true));
         }
     }