ipvlan: fix building with modular IPV6

We no longer depend on IPV6, but that now causes a link error with
CONFIG_IPV6=m and CONFIG_IPVLAN=y:

drivers/net/ipvlan/ipvlan_core.o: In function `ipvlan_queue_xmit':
ipvlan_core.c:(.text+0x1440): undefined reference to `ip6_route_output_flags'
drivers/net/ipvlan/ipvlan_core.o: In function `ipvlan_l3_rcv':
ipvlan_core.c:(.text+0x1818): undefined reference to `ip6_route_input_lookup'

This adds back the dependency on IPV6, with the option of building without
IPV6, but forcing IPVLAN to be a module when IPV6 is a module.

Fixes: 94333fac ("ipvlan: drop ipv6 dependency")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>