libconfuse: needs host-flex

Fixes:
http://autobuild.buildroot.net/results/63f/63f6f6204058932b02f79eaaa44eb4555c5bbb1f/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>