[healthkit] Update for beta 4

Fix introspection failure

[FAIL] HKWorkoutEvent conforms to NSCopying but does not implement INSCopying

as conformance for NSCopying was added in beta 4 (it's the only change)

HealthKit-iOS-Beta4.md:-@interface HKWorkoutEvent : NSObject <NSSecureCoding>
HealthKit-iOS-Beta4.md:+@interface HKWorkoutEvent : NSObject <NSSecureCoding, NSCopying>

src/healthkit.cs

@@ -1534,7 +1534,7 @@ namespace XamCore.HealthKit {
 	[iOS (8,0)]
 	[BaseType (typeof (NSObject))]
 	[DisableDefaultCtor]
-	public interface HKWorkoutEvent : NSSecureCoding {
+	public interface HKWorkoutEvent : NSSecureCoding, NSCopying {
 		[Export ("type")]
 		HKWorkoutEventType Type { get; }