package/matchbox-startup-monitor: invert dependency chain

Previously, matchbox-startup-monitor was forcibly enabled whenever
startup-notification was. Now that it has a prompt, it merely defaults
to 'y' in this case, leaving the user the possibility to disable it.
This is not very logical.

Instead, invert the logic, and select startup-notification from
matchbox-startup-monitor.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>