Remove prefix 'CONFIG_' from CONFIG_IPV6

Fixes: ba7d7e26 ("net/rds/Kconfig: RDS should depend on IPV6")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>