Fixed Notification type to work on both Linux and OS X
Fixed Notification type to work on both Linux and OS X

- The previous commit broke the Notification struct for OS X builds

ibm-bluemix-mobile-services/bms-pushnotifications-serversdk-swift,ibm-bluemix-mobile-services/bluemix-pushnotifications-swift-sdk,ibm-bluemix-mobile-services/bluemix-pushnotifications-swift-sdk,ibm-bluemix-mobile-services/bms-pushnotifications-serversdk-swift