package/matchbox-panel: fix WiFi monitoring

matchbox-panel can build a simple applet to monitor the WiFi. For that,
it tries to see if libiwl is available; if so, WiFi monitoring is
enabled, otherwise it is disabled; there is not option to explicitly
enable or disable it.

Currently, matchbox-panel depends on the wireless-tools package.
However, that's not enough, since the library will only be installed if
explicitly enabled in the menuconfig *and* this is not a static build.

Change the dependency to be on the library, not the package.

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>