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>

package/sane-backends/sane-backends.mk

@@ -39,6 +39,7 @@ SANE_BACKENDS_CONF_OPTS += --enable-avahi
 endif
 
 ifeq ($(BR2_PACKAGE_NETSNMP),y)
+SANE_BACKENDS_CONF_ENV += ac_cv_path_SNMP_CONFIG_PATH=$(STAGING_DIR)/usr/bin/net-snmp-config
 SANE_BACKENDS_DEPENDENCIES += netsnmp
 else
 SANE_BACKENDS_CONF_OPTS += --without-snmp