Fix:Yuzu freezing when stopping emulation on Linux

While working on Windows, stopping emulation caused Yuzu to freeze on Linux. That includes closing the emulator window while a game was running, stopping the game running from Emulation>Stop and restarting it from Emulation>Restart. What happened was the thread stayed forever in a waiting state.

src/video_core/shader/async_shaders.cpp

@@ -64,6 +64,7 @@ void AsyncShaders::FreeWorkers() {
 
 void AsyncShaders::KillWorkers() {
     is_thread_exiting.store(true);
+    cv.notify_all();
     for (auto& thread : worker_threads) {
         thread.detach();
     }