package/matchbox-keyboard: enforce disabling support for cairo

We usually explicitly enforce disabling optional support when it is
possible.

Do so for Cairo.

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>