selftests: net: add missing config for NF_TARGET_TTL

amt test uses the TTL iptables module:

  ip netns exec "${RELAY}" iptables -t mangle -I PREROUTING \
  	-d 239.0.0.1 -j TTL --ttl-set 2

Fixes: c08e8bae ("selftests: add amt interface selftest script")
Link: https://lore.kernel.org/r/20240131165605.4051645-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/config

@@ -33,6 +33,7 @@ CONFIG_IP6_NF_NAT=m
 CONFIG_IP6_NF_RAW=m
 CONFIG_IP_NF_NAT=m
 CONFIG_IP_NF_RAW=m
+CONFIG_IP_NF_TARGET_TTL=m
 CONFIG_IPV6_GRE=m
 CONFIG_IPV6_SEG6_LWTUNNEL=y
 CONFIG_L2TP_ETH=m