crypto/nss: some nss_ctx_init() params made const
    
    This patch fixes compilation issues introduced in 8aa0dfd :
    as `key` parameter of sss_encrypt() and sss_decrypt() became const,
    changes in signature of nss_ctx_init() were required to follow up.
    For more details see https://github.com/SSSD/sssd/pull/846
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>