linux-zigbee: fix build with gcc 7

Disable -Werror to avoid the fatal result of new gcc 7 format string warnings.

Fixes:
http://autobuild.buildroot.net/results/29c/29c72bc38042305310576be945c721b2fad95894/
http://autobuild.buildroot.net/results/a7d/a7d38d72834b94291eaff159da277b11e2f9d63a/
http://autobuild.buildroot.net/results/cfe/cfed5176075f0cb9e2f56ebef10f5d6c352baf10/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/linux-zigbee/linux-zigbee.mk

@@ -15,6 +15,7 @@ LINUX_ZIGBEE_AUTORECONF = YES
 
 LINUX_ZIGBEE_CONF_OPTS = \
 	--disable-manpages \
+	--disable-werror \
 	--with-leasefile="$(call qstrip,$(BR2_PACKAGE_LINUX_ZIGBEE_LEASEFILE))"
 
 ifeq ($(BR2_PACKAGE_LINUX_ZIGBEE_TESTS),y)