sane-backends: specify explicit path to net-snmp-config

Otherwise if the host distro has net-snmp devel files sane-backends will
find /usr/bin/net-snmp-config first and the build will break. Fixes:
http://autobuild.buildroot.net/results/c38/c381c316907964787ebe1a0807399022f15d8158/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>