hle: kernel: Move ServiceThread ownership to KernelCore.

- Fixes a circular dependency which prevented threads from being released on shutdown.

src/core/hle/kernel/server_session.h

@@ -167,7 +167,7 @@ private:
     std::string name;
 
     /// Thread to dispatch service requests
-    std::unique_ptr<ServiceThread> service_thread;
+    std::weak_ptr<ServiceThread> service_thread;
 };
 
 } // namespace Kernel