Add a missing virtual destructor to CollectionChangeCallback
Add a missing virtual destructor to CollectionChangeCallback

This resulted in the destructor for the callback object passed to
`add_notification_callback()` not being called, potentially resulting in memory
leaks when removing notification callbacks.

realm/realm-object-store,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-core,realm/realm-core