Fix adjusting volume by scrolling on panel plugin button

With Gtk+3 one needs to add GDK_SCROLL_MASK to the widget event list, as done in panel-plugins/xfce4-mpc-plugin@fdcbe645 and in https://gitlab.xfce.org/panel-plugins/xfce4-pulseaudio-plugin/-/blob/master/panel-plugin/pulseaudio-button.c#L147