fixes #5325 use setImportDir instead of addImportDir so that the defa…

…ult dir "" (empty string) is not used (as it triggers warning on windows with openbasedir)

This fixes error Warning:is_file(): open_basedir restriction in effect. File(/plugins/Zeitgeist/stylesheets/less/_colors.less.less) is not within the allowed path(s): (C:\Inetpub\temp;D:\Inetpub) in D:\Inetpub\WWWRoot\piwik\vendor\leafo\lessphp\lessc.inc.php on line 82

core/AssetManager/UIAssetMerger/StylesheetUIAssetMerger.php

@@ -30,8 +30,10 @@ function __construct($mergedAsset, $assetFetcher, $cacheBuster)
 
     protected function getMergedAssets()
     {
-        $this->lessCompiler->addImportDir(PIWIK_USER_PATH);
-        return $this->lessCompiler->compile($this->getConcatenatedAssets());
+        // note: we're using setImportDir on purpose (not addImportDir)
+        $this->lessCompiler->setImportDir(PIWIK_USER_PATH);
+        $concatenatedAssets = $this->getConcatenatedAssets();
+        return $this->lessCompiler->compile($concatenatedAssets);
     }
 
     /**