Kernel/Mutex: Add a missing return in Mutex::Release

Fixes a regression in #3042.
Closes #3079

src/core/hle/kernel/mutex.cpp

@@ -88,6 +88,8 @@ ResultCode Mutex::Release(Thread* thread) {
         WakeupAllWaitingThreads();
         Core::System::GetInstance().PrepareReschedule();
     }
+    
+    return RESULT_SUCCESS;
 }
 
 void Mutex::AddWaitingThread(SharedPtr<Thread> thread) {