package/matchbox-panel: add support for dnotify

Using dnotify, matchbox-panel can automatically reload its menu.
dnotify requires a kernel >=2.4, which we can consider to be
virtually for granted.

So, forcibly use dnotify.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>