[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>

net/xfrm/xfrm_state.c

@@ -1749,6 +1749,7 @@ void km_policy_expired(struct xfrm_policy *pol, int dir, int hard, u32 pid)
 }
 EXPORT_SYMBOL(km_policy_expired);
 
+#ifdef CONFIG_XFRM_MIGRATE
 int km_migrate(struct xfrm_selector *sel, u8 dir, u8 type,
 	       struct xfrm_migrate *m, int num_migrate)
 {
@@ -1768,6 +1769,7 @@ int km_migrate(struct xfrm_selector *sel, u8 dir, u8 type,
 	return err;
 }
 EXPORT_SYMBOL(km_migrate);
+#endif
 
 int km_report(u8 proto, struct xfrm_selector *sel, xfrm_address_t *addr)
 {