[XFRM]: Do not define km_migrate() if !CONFIG_XFRM_MIGRATE

In include/net/xfrm.h we find :

#ifdef CONFIG_XFRM_MIGRATE
extern int km_migrate(struct xfrm_selector *sel, u8 dir, u8 type,
                      struct xfrm_migrate *m, int num_bundles);
...
#endif

We can also guard the function body itself in net/xfrm/xfrm_state.c
with same condition.

(Problem spoted by sparse checker)
make C=2 net/xfrm/xfrm_state.o
...
net/xfrm/xfrm_state.c:1765:5: warning: symbol 'km_migrate' was not declared. Should it be static?
...

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>