skypeforlinux: support app indicator

In contrast to e.g. Telegram or Slack, Skype does not show an app
indicator in the GNOME tray. This is quite annoying, since Skype will
continue to run in the background when its main window is closed, but
there is no way to access it.

This change adds libappindicator-gtk3 to the rpath to enable app
indicator support.