zbar: needs kernel headers >= 3.17

v4l2_query_ext_ctrl has been added in kernel 3.17:
https://github.com/torvalds/linux/commit/5082c2417841e64df975789011e182ce99a9dacd

Fixes:
 - http://autobuild.buildroot.org/results/d7b244cf9488eafb59ba8575f17884f4f8512db1



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>