Update demos

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

demos/src/sm4_cbc_mac_demo.c

@@ -0,0 +1,42 @@
+/*
+ *  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/sm4_cbc_mac.h>
+
+
+int main(int argc, char **argv)
+{
+	SM4_CBC_MAC_CTX ctx;
+	uint8_t key[16] = {0};
+	uint8_t data[16 * 3] = {0};
+	uint8_t mac[16] = {0};
+	size_t i;
+
+	if (sizeof(data) % SM4_BLOCK_SIZE != 0) {
+		fprintf(stderr, "CBC_MAC data length error\n");
+		goto err;
+	}
+
+	sm4_cbc_mac_init(&ctx, key);
+	sm4_cbc_mac_update(&ctx, data, sizeof(data));
+	sm4_cbc_mac_finish(&ctx, mac);
+
+	printf("cbc_mac = ");
+	for (i = 0; i < sizeof(mac); i++) {
+		printf("%02x", mac[i]);
+	}
+	printf("\n");
+
+err:
+	return 0;
+}