Use global namespace for cURL functions

[yani]

When cURL is not installed, it will throw an error that the function Office365\PHP\Client\Runtime\Utilities\curn_init() does not exist at line 96 in src/Runtime/Utilities/Requests.php. Both the namespace and the global namespace are checked, but the error gets thrown in the namespace. It would be better to force the global namespace so that this error will not get misinterpreted for a missing internal method.