xfrm: Remove the repeated declaration

Function 'xfrm_parse_spi' is declared twice, so remove the
repeated declaration.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>