PHPSDK-172: Add BILLINK to SHOPPING_CART_REQUIRED_GATEWAYS constant (#377)

Author: danielcivit

src/Api/Transactions/Gateways.php

@@ -26,5 +26,6 @@ class Gateways
         'ZINIA_IN3',
         'BNPL_OB',
         'BNPL_MF',
+        'BILLINK',
     );
 }