use @IPV6 instead of +kmod-ipv6

kmod-ipv6 is obsolete.
The symbol IPV6 is a global config symbol, don't select it because it
will override the user decision of IPV6 as all other package in base do
it. A select (+) will also create a recursive dependency error.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>