package/matchbox: xlib-libXcursor is an optional dependency

matchbox selects xlib-libXcursor, but does not build-depend on it.

But xlib-libXcursor is only an optional dependency.

Fix that by removing the select, and properly build-depend on it when
it is enabled.

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>