fou: dead build rule for fou6.o

[Valentin Rothberg]

Hi Arnd,

your commit fabb13db448e ("fou: add Kconfig options for IPv6 support")
has shown up in today's linux-next. ./scripts/checkkconfigsymbols.py
warned because of the following statement in net/ipv6/Makefile:

+obj-$(CONFIG_NET_FOU_IPV6_TUNNELS) += fou6.o

The referenced Kconfig option does not exist, but I assume the right
option is 'IPV6_FOU_TUNNEL'?  In case yes, the modified #if block in
the commit could be removed (redundant check).

Best regards,
 Valentin