selftests: forwarding: Add missing multicast routing config entries

The two tests that make use of multicast routig (router.sh and
router_multicast.sh) are currently failing in the netdev CI because the
kernel is missing multicast routing support.

Fix by adding the required config entries.

Fixes: 6d4efada ("selftests: forwarding: Add multicast routing test")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240208165538.1303021-1-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/config

@@ -9,6 +9,13 @@ CONFIG_CGROUP_BPF=y
 CONFIG_DUMMY=m
 CONFIG_IPV6=y
 CONFIG_IPV6_GRE=m
+CONFIG_IPV6_MROUTE=y
+CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y
+CONFIG_IPV6_PIMSM_V2=y
+CONFIG_IP_MROUTE=y
+CONFIG_IP_MROUTE_MULTIPLE_TABLES=y
+CONFIG_IP_PIMSM_V1=y
+CONFIG_IP_PIMSM_V2=y
 CONFIG_MACVLAN=m
 CONFIG_NET_ACT_CT=m
 CONFIG_NET_ACT_MIRRED=m