package/matchbox-fakekey: add missing dependency

matchbox-fakekey build-depends on xlib_libXtst, but does not select it.

Fix that.

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>