Fix build on Mac with -DICINGA2_UNITY_BUILD=OFF -DICINGA2_WITH_LIVEST…

…ATUS=ON

error: no matching function for call to 'intrusive_ptr_release'
...
candidate function not viable: cannot convert argument of incomplete type 'icinga::Notification *' to 'Object *' for 1st argument
void intrusive_ptr_release(Object *object);

lib/icinga/notification.hpp

@@ -55,6 +55,7 @@ class ApplyRule;
 struct ScriptFrame;
 class Host;
 class Service;
+class UserGroup;
 
 /**
  * An Icinga notification specification.
@@ -73,7 +74,7 @@ class Notification final : public ObjectImpl<Notification>
 	intrusive_ptr<NotificationCommand> GetCommand() const;
 	TimePeriod::Ptr GetPeriod() const;
 	std::set<User::Ptr> GetUsers() const;
-	std::set<UserGroup::Ptr> GetUserGroups() const;
+	std::set<intrusive_ptr<UserGroup>> GetUserGroups() const;
 
 	void UpdateNotificationNumber();
 	void ResetNotificationNumber();

lib/icinga/usergroup.hpp

@@ -4,6 +4,7 @@
 #define USERGROUP_H
 
 #include "icinga/i2-icinga.hpp"
+#include "icinga/notification.hpp"
 #include "icinga/usergroup-ti.hpp"
 #include "icinga/user.hpp"