package/syslog-ng: fix libnet detection

Fixes
http://autobuild.buildroot.net/results/e71/e718aab76d41c9c1aa5f9e3e74fb48b63038f254/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/syslog-ng/syslog-ng.mk

@@ -71,7 +71,9 @@ endif
 
 ifeq ($(BR2_PACKAGE_LIBNET),y)
 SYSLOG_NG_DEPENDENCIES += libnet
-SYSLOG_NG_CONF_OPTS += --enable-spoof-source
+SYSLOG_NG_CONF_OPTS += \
+	--with-libnet=$(STAGING_DIR)/usr/bin \
+	--enable-spoof-source
 else
 SYSLOG_NG_CONF_OPTS += --disable-spoof-source
 endif