selftests: ovpn: add nftables config dependencies for test-mark

selftests: ovpn: add nftables config dependencies for test-mark

test-mark.sh installs nftables rules in an inet/filter output chain and
verifies packet drops via nft counters. In vmksft this can fail when the
nftables core is not enabled by the ovpn selftest config.

Add the missing kernel options required by this test:
- CONFIG_NETFILTER
- CONFIG_NF_TABLES
- CONFIG_NF_TABLES_INET

Fixes: 7b80d8a33500 ("selftests: ovpn: add test for the FW mark feature")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/all/20260319124114.42f91f72@kernel.org/
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>