fix: add X-Deepin-Singleton flag to desktop file

Add X-Deepin-Singleton=true to desktop file to mark the
application as a singleton app.

为单例应用的desktop文件添加X-Deepin-Singleton=true标识。

Log: 添加单例应用标识
PMS: TASK-392841
Influence: AM能识别单例应用，Treeland不再为单例应用显示快速启动画面

Author: add-uos

deepin-screen-recorder.desktop

@@ -13,6 +13,7 @@ Type=Application
 X-Deepin-ManualID=deepin-screen-recorder
 X-Deepin-Vendor=deepin
 X-Deepin-TurboType=dtkwidget
+X-Deepin-Singleton=true
 NotShowIn=DeepinTablet
 
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