package/network/utils/iptables: fix PKG_CPE_ID

package/network/utils/iptables: fix PKG_CPE_ID

cpe:/a:netfilter:iptables is the correct CPE ID for iptables:
https://nvd.nist.gov/products/cpe/search/results?keyword=cpe:2.3:a:netfilter:iptables

Fixes: c61a2395140d92cdd37d3d6ee43a765427e8e318 (add PKG_CPE_ID ids to package and tools)
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15297
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 6e5edec159e2dc468607a5b9179f722857a38421)
Link: https://github.com/openwrt/openwrt/pull/15881
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>