Add missing QPointer

The Compositor is destroyed after the Workspace, so the placeholder has
shorter lifespan. We need to guard the destructor of EffectScreenImpl
from accessing a dangling pointer. For what it's worth, the destructor
of EffectScreenImpl was written with QPointer in mind, but it was never
added.

src/effects.h

@@ -374,7 +374,7 @@ public:
     static EffectScreenImpl *get(Output *output);
 
 private:
-    Output *m_platformOutput;
+    QPointer<Output> m_platformOutput;
 };
 
 class EffectWindowImpl : public EffectWindow