matchbox-panel: specify optional dependency on wireless_tools

Make sure build is reproducible and doesn't depend on the order.

There is no corresponding configure option, but configure looks for
iwlib.h

Cc: Jean-Paul Etienne <fractalclone@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>