xfrm: fix xfrm_dev_event() missing when compile without CONFIG_XFRM_OFFLOAD

In commit d77e38e6 ("xfrm: Add an IPsec hardware offloading API") we
make xfrm_device.o only compiled when enable option CONFIG_XFRM_OFFLOAD.
But this will make xfrm_dev_event() missing if we only enable default XFRM
options.

Then if we set down and unregister an interface with IPsec on it. there
will no xfrm_garbage_collect(), which will cause dev usage count hold and
get error like:

unregister_netdevice: waiting for <dev> to become free. Usage count = 4

Fixes: d77e38e6 ("xfrm: Add an IPsec hardware offloading API")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>