notification_proxy: Make np_observe_notifications() atomic
notification_proxy: Make np_observe_notifications() atomic

Otherwise the notification callback might fire before all notifications that should be observed have been
registered. This way the callback will only be called after _all_ notifications have been registered.

libimobiledevice-win32/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice