net/xfrm/xfrm_output.c: move EXPORT_SYMBOL

Fix checkpatch warning:
"WARNING: EXPORT_SYMBOL(foo); should immediately follow its function/variable"

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>