ipv6: sr: select DST_CACHE by default

When CONFIG_IPV6_SEG6_LWTUNNEL is selected, automatically select DST_CACHE.
This allows to remove multiple ifdefs.

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>