xtables-addons: add PKG_LICENSE_FILES and use SPDX

Add PKG_LICENSE_FILES. Use SPDX.

Signed-off-by: Nick Hainke <vincent@systemli.org>
(cherry picked from commit b4d00c1c20)

net/xtables-addons/Makefile

@@ -1,9 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0-only
 #
 # Copyright (C) 2009-2013 OpenWrt.org
 #
-# This is free software, licensed under the GNU General Public License v2.
-# See /LICENSE for more information.
-#
 
 include $(TOPDIR)/rules.mk
 include $(INCLUDE_DIR)/kernel.mk
@@ -16,11 +14,13 @@ PKG_HASH:=5e36ea027ab15a84d9af1f3f8e84a78b80a617093657f08089bd44657722f661
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=https://inai.de/files/xtables-addons/
 PKG_BUILD_DEPENDS:=iptables
+
 PKG_INSTALL:=1
 PKG_BUILD_PARALLEL:=1
 
 PKG_MAINTAINER:=Jo-Philipp Wich <jo@mein.io>
 PKG_LICENSE:=GPL-2.0
+PKG_LICENSE_FILES:=COPYING
 
 PKG_FIXUP:=autoreconf
 PKG_ASLR_PIE:=0