[CloudKit] Xcode 9.3 beta 1 changes. (#3321)

src/CloudKit/Enums.cs

@@ -81,6 +81,7 @@ namespace XamCore.CloudKit
 		[iOS (10,0), TV (10,0), Mac (10,12)] ManagedAccountRestricted = 32,
 		[iOS (10,0), TV (10,0), Mac (10,12)] ParticipantMayNeedVerification = 33,
 		[iOS (11,0), TV (11,0), Mac (10,13), Watch (4,0)] ResponseLost = 34,
+		[iOS (11,3), TV (11,3), Mac (10,13), Watch (4,3)] AssetNotAvailable = 35,
 	}
 
 	// NSInteger -> CKModifyRecordsOperation.h