ip_gre: CONFIG_IPV6_MODULE support

ipv6 can be a module, we should test CONFIG_IPV6 and CONFIG_IPV6_MODULE
to enable ipv6 bits in ip_gre.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>