selftests: netfilter: fix current year

use date %Y instead of %G to read current year
Problem appeared when running lkp-tests on 01/01/2021

Fixes: 48d072c4 ("selftests: netfilter: add time counter check")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>