selftests: net: add missing config for nftables-backed iptables

Modern OSes use iptables implementation with nf_tables as a backend,
e.g.:

$ iptables -V
iptables v1.8.8 (nf_tables)

Pablo points out that we need CONFIG_NFT_COMPAT to make that work,
otherwise we see a lot of:

  Warning: Extension DNAT revision 0 not supported, missing kernel module?

with DNAT being just an example here, other modules we need
include udp, TTL, length etc.

Link: https://lore.kernel.org/r/20240126201308.2903602-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/config

@@ -60,6 +60,7 @@ CONFIG_NET_SCH_HTB=m
 CONFIG_NET_SCH_FQ=m
 CONFIG_NET_SCH_ETF=m
 CONFIG_NET_SCH_NETEM=y
+CONFIG_NFT_COMPAT=m
 CONFIG_NF_FLOW_TABLE=m
 CONFIG_PSAMPLE=m
 CONFIG_TCP_MD5SIG=y