configs: Enable IP_VS_IPV6 (for loadbalancing)

The Raspberry Pi defconfigs enable suport for IP Virtual Servers with
many of the optional features, but for some reason exclude the IPV6
support. The relevant Kconfig file says "Say Y if unsure", and the
change only makes a few modules a bit larger (~30kB), so most users
should see no difference.

See: https://github.com/raspberrypi/linux/issues/2860



Signed-off-by: Phil Elwell <phil@raspberrypi.com>