[libnotificationmanager] Fix memory leak

In an &operator= we already have an object. It will have the d-pointer
already allocated.

If we create a new private implementation our old one leaks

libnotificationmanager/notification.cpp

@@ -449,7 +449,7 @@ Notification::Notification(Notification &&other) noexcept
 
 Notification &Notification::operator=(const Notification &other)
 {
-    d = new Private(*other.d);
+    *d = *other.d;
     return *this;
 }