Mutex: Replace g_mutex_held_locks with a set inside Thread

src/core/hle/kernel/thread.cpp

@@ -40,6 +40,9 @@ static Thread* current_thread;
 static const u32 INITIAL_THREAD_ID = 1; ///< The first available thread id at startup
 static u32 next_thread_id; ///< The next available thread id
 
+Thread::Thread() {
+}
+
 Thread* GetCurrentThread() {
     return current_thread;
 }