Fix ssh/sshd failing in Windows 7 due to a flag use in cng_digest

Avoid using BCRYPT_HASH_REUSABLE_FLAG in cng_digest.c file which was
introduced in Windows 8 and not supported in previous OS like Windows 7.

Author: quamrulmina

contrib/win32/win32compat/cng_digest.c

@@ -144,7 +144,7 @@ struct ssh_digest_ctx *
 		return NULL;
 	}
 
-	if ((hr = BCryptCreateHash(ret->cng_alg_handle, &(ret->hash_handle), NULL, 0, NULL, 0, BCRYPT_HASH_REUSABLE_FLAG)) != S_OK)
+	if ((hr = BCryptCreateHash(ret->cng_alg_handle, &(ret->hash_handle), NULL, 0, NULL, 0, 0)) != S_OK)
 	{
 		BCryptCloseAlgorithmProvider(ret->cng_alg_handle, 0);
 		free(ret);