fix pdf export cache directory (#650)

src/Utils/HtmlToPdfConverter.php

@@ -0,0 +1,20 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Utils;
+
+interface HtmlToPdfConverter
+{
+    /**
+     * Returns the binary content of the PDF, which can be saved as file or send via Reponse.
+     * @param string $html
+     * @return mixed
+     */
+    public function convertToPdf(string $html);
+}