Add SM2 demos

demos/sm2/sm2_private_key_demo.c

@@ -0,0 +1,22 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/sm2.h>
+
+int main(void)
+{
+	SM2_KEY sm2_key;
+	char *password = "123456";
+
+	if (sm2_key_generate(&sm2_key) != 1) {
+		fprintf(stderr, "error\n");
+		return 1;
+	}
+
+	if (sm2_private_key_info_encrypt_to_pem(&sm2_key, password, stdout) != 1) {
+		fprintf(stderr, "error\n");
+		return 1;
+	}
+
+	return 0;
+}