Use DIRECTORY_SEPARATOR in getBasePath()

Hard-coded forward slash resulted in incorrect path on Windows

CRM/Utils/System/Joomla.php

@@ -529,7 +529,7 @@ public function getJVersion($joomlaBase) {
    */
   public function getBasePath() {
     global $civicrm_root;
-    $joomlaPath = explode('/administrator', $civicrm_root);
+    $joomlaPath = explode(DIRECTORY_SEPARATOR . 'administrator', $civicrm_root);
     $joomlaBase = $joomlaPath[0];
     return $joomlaBase;
   }