Fix NSDistributedNotificationCenter.DefaultCenter return value

- As we can't change NSDistributedNotificationCenter.DefaultCenter return value outside of XAMCORE_4_0 add NSDistributedNotificationCenter.GetDefaultCenter () and point people to that for now.
- Fixed in XAMCORE_4_0 without the hack

src/foundation.cs

@@ -10255,7 +10255,15 @@ namespace Foundation
 	interface NSDistributedNotificationCenter {
 		[Static]
 		[Export ("defaultCenter")]
+#if XAMCORE_4_0
+		NSDistributedNotificationCenter DefaultCenter { get; }
+#else
+		NSDistributedNotificationCenter GetDefaultCenter ();
+
+		[Advice ("Use 'GetDefaultCenter ()' for a strongly typed version.")]
+		[Wrap ("GetDefaultCenter ()", IsVirtual = true)]
 		NSObject DefaultCenter { get; }
+#endif
 
 		[Export ("addObserver:selector:name:object:suspensionBehavior:")]
 		void AddObserver (NSObject observer, Selector selector, [NullAllowed] string notificationName, [NullAllowed] string notificationSenderc, NSNotificationSuspensionBehavior suspensionBehavior);