package/sane-backends: remove unsafe headers path

Fixes:
http://autobuild.buildroot.net/results/deb/deb3d45416633b9217d3cc09664273687c809464/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>