crypto: arm/curve25519 - include <linux/scatterlist.h>

Building ARM allmodconfig leads to the following warnings:

arch/arm/crypto/curve25519-glue.c:73:12: error: implicit declaration of function 'sg_copy_to_buffer' [-Werror=implicit-function-declaration]
arch/arm/crypto/curve25519-glue.c:74:9: error: implicit declaration of function 'sg_nents_for_len' [-Werror=implicit-function-declaration]
arch/arm/crypto/curve25519-glue.c:88:11: error: implicit declaration of function 'sg_copy_from_buffer' [-Werror=implicit-function-declaration]

Include <linux/scatterlist.h> to fix such warnings

Reported-by: Olof's autobuilder <build@lixom.net>
Fixes: 0c3dc787

 ("crypto: algapi - Remove skbuff.h inclusion")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>