drivers/net: macvtap and tun depend on INET

These drivers now call ipv6_proxy_select_ident(), which is defined
only if CONFIG_INET is enabled.  However, they have really depended
on CONFIG_INET for as long as they have allowed sending GSO packets
from userland.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: f43798c2 ("tun: Allow GSO using virtio_net_hdr")
Fixes: b9fb9ee0 ("macvtap: add GSO/csum offload support")
Fixes: 5188cd44

 ("drivers/net, ipv6: Select IPv6 fragment idents for virtio UFO packets")
Signed-off-by: David S. Miller <davem@davemloft.net>