Update demos

To build demos, run
`cmake .. -DENABLE_DEMOS=ON`

demos/src/sdf_rand_demo.c

@@ -0,0 +1,47 @@
+/*
+ *  Copyright 2014-2023 The GmSSL Project. All Rights Reserved.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the License); you may
+ *  not use this file except in compliance with the License.
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ */
+
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/sdf.h>
+#include <gmssl/error.h>
+
+
+int main(void)
+{
+	int ret = -1;
+	char *so_path = "libsdf_dummy.so";
+	SDF_DEVICE dev;
+	uint8_t buf[32];
+
+	if (sdf_load_library(so_path, NULL) != 1) {
+		error_print();
+		return -1;
+	}
+
+	if (sdf_open_device(&dev) != 1) {
+		error_print();
+		goto err;
+	}
+
+	if (sdf_rand_bytes(&dev, buf, sizeof(buf)) != 1) {
+		error_print();
+		goto err;
+	}
+
+	format_bytes(stdout, 0, 0, "sdf_rand_bytes", buf, sizeof(buf));
+
+	ret = 0;
+err:
+	sdf_close_device(&dev);
+	sdf_unload_library();
+	return ret;
+}