wincng: Made data parameter to hash update function constant
wincng: Made data parameter to hash update function constant

Initially reported by Bob Kast as "formal parameter must be const
since it is used in contexts where the actual parameter may be const".
Thanks a lot.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2