gnetworkmonitorbase: Add missing notify::connectivity signal

The connectivity is directly derived from the `is_available` variable,
so if that changes, we need to notify of a change in `connectivity` too.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3803

gio/gnetworkmonitorbase.c

@@ -443,6 +443,7 @@ emit_network_changed (gpointer user_data)
     {
       monitor->priv->is_available = is_available;
       g_object_notify (G_OBJECT (monitor), "network-available");
+      g_object_notify (G_OBJECT (monitor), "connectivity");
     }
 
   g_signal_emit (monitor, network_changed_signal, 0, is_available);