src: get source code license header in sync with current licensing terms

Since (3956761 license: upgrade to GPLv2+), we upgraded to GPLv2+,
propagate that changes to src/iftable.c and src/rtnl.c

Reported-by: Thomas Woerner <twoerner@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>