staging: skein: fixed sparse warnings related to static declarations

drivers/staging/skein/skein_generic.c:30:5: warning: symbol 'skein256_update' was not declared. Should it be static?
drivers/staging/skein/skein_generic.c:65:5: warning: symbol 'skein512_update' was not declared. Should it be static?
drivers/staging/skein/skein_generic.c:100:5: warning: symbol 'skein1024_update' was not declared. Should it be static?

Signed-off-by: Niklas Svensson <nks@flawful.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>