amt: add IPV6 Kconfig dependency

This driver cannot be built-in if IPV6 is a loadable module:

x86_64-linux-ld: drivers/net/amt.o: in function `amt_build_mld_gq':
amt.c:(.text+0x2e7d): undefined reference to `ipv6_dev_get_saddr'

Add the idiomatic Kconfig dependency that all such modules
have.

Fixes: b9022b53 ("amt: add control plane of amt interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>