build: always set CONFIG_IPV6

build: always set CONFIG_IPV6

Disabling this build tunable breaks build and seems unrealistically
likely to be fixed.

This patch sets the related CONFIG to always true and removes the
config prompt, keeping the change minimal, and, should !CONFIG_IPV6 ever
be fixed, easy to revert.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
Acked-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Paul Spooren <mail@aparcar.org>
Acked-by: Josef.Schlehofer <pepe.schlehofer@gmail.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>