net: xfrm: xfrm_policy: silence compiler warning

Fix below compiler warning:

net/xfrm/xfrm_policy.c:1644:12: warning: ‘xfrm_dst_alloc_copy’ defined but not used [-Wunused-function]

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>