Update demos

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

demos/src/sm3_hmac_demo.c

@@ -0,0 +1,38 @@
+/*
+ *  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/sm3.h>
+
+
+int main(void)
+{
+	unsigned char key[16] = {
+		0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,
+		0x01,0xf2,0x03,0x04,0x05,0x06,0x07,0x08,
+	};
+	unsigned char data[16] = {
+		0x31,0x32,0x33,0x34,0x35,0x36,0x37,0x38,
+		0x31,0x32,0x33,0x34,0x35,0x36,0x37,0x38,
+	};
+	unsigned char hmac[32] = {0};
+	int i;
+
+	sm3_hmac(key, sizeof(key), data, sizeof(data), hmac);
+
+	printf("hmac: ");
+	for (i = 0; i < sizeof(hmac); i++) {
+		printf("%02x", hmac[i]);
+	}
+	printf("\n");
+
+	return 0;
+}