[XFRM]: xfrm_state_clone() should be static, not exported

xfrm_state_clone() is not used outside of net/xfrm/xfrm_state.c
There is no need to export it.

Spoted by sparse checker.
   CHECK   net/xfrm/xfrm_state.c
net/xfrm/xfrm_state.c:1103:19: warning: symbol 'xfrm_state_clone' was not
declared. Should it be static?

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