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>

package/matchbox/matchbox-panel/matchbox-panel.mk

@@ -22,6 +22,10 @@ else
 MATCHBOX_PANEL_CONF_OPTS += --disable-startup-notification
 endif
 
+ifeq ($(BR2_PACKAGE_WIRELESS_TOOLS),y)
+MATCHBOX_PANEL_DEPENDENCIES += wireless_tools
+endif
+
 ################################################################################
 
 $(eval $(autotools-package))