staging: skein: fix sparse warning for static declarations

This patch fixes the following sparse warnings:

skein_block.c:43:6: warning: symbol 'skein_256_process_block' was not declared. Should it be static?
skein_block.c:252:6: warning: symbol 'skein_512_process_block' was not declared. Should it be static?
skein_block.c:483:6: warning: symbol 'skein_1024_process_block' was not declared. Should it be static?

by including the header skein_block.h, which contains the declarations in
question.

Signed-off-by: James A Shackleford <shack@linux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>