MDL-20438 make sure that the curl class has been included

The filelib is usually loaded already. However, during some steps of the
upgrade the curl class must be available to perform fetches of updates
and the library does not need to be loaded yet.

lib/pluginlib.php

@@ -30,6 +30,8 @@
 
 defined('MOODLE_INTERNAL') || die();
 
+require_once($CFG->libdir.'/filelib.php');  // curl class needed here
+
 /**
  * Singleton class providing general plugins management functionality
  */