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.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>