initPaths: set include path before any owncloud function is called

Ensure we can find basic owncloud classes while detecting the owncloud
paths in function initPaths by add OC::$SERVERROOT . '/lib' to include
path before any owncloud function is called.

This fix a bug with XAMPP that cannot find the class OC_Config in
function OC_Request::scriptName when called from function
OC_Base::initPaths.