package/matchbox-common: fix dependencies

matchbox-common can be used by both matchbox-desktop and matchbox-panel,
so it needs to depend on either, not both.

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