selftests: net: add missing config for amt.sh

Test needs IPv6 multicast. smcroute currently crashes when trying
to install a route in a kernel without IPv6 multicast.

Fixes: c08e8bae ("selftests: add amt interface selftest script")
Link: https://lore.kernel.org/r/20240509161919.3939966-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/config

@@ -30,6 +30,7 @@ CONFIG_IP_GRE=m
 CONFIG_NETFILTER=y
 CONFIG_NETFILTER_ADVANCED=y
 CONFIG_NF_CONNTRACK=m
+CONFIG_IPV6_MROUTE=y
 CONFIG_IPV6_SIT=y
 CONFIG_IP_DCCP=m
 CONFIG_NF_NAT=m