xfrm: fix XFRMA_LASTUSED comment

It is a __u64, internally time64_t.

Fixes: bf825f81 ("xfrm: introduce basic mark infrastructure")
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>