Add SM2 demos

demos/sm2/sm2_keygen_demo.c

@@ -0,0 +1,19 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/sm2.h>
+
+int main(void)
+{
+	SM2_KEY sm2_key;
+
+	if (sm2_key_generate(&sm2_key) != 1) {
+		fprintf(stderr, "error\n");
+		return 1;
+	}
+
+	sm2_key_print(stdout, 0, 0, "SM2PrivateKey", &sm2_key);
+	sm2_public_key_print(stdout, 0, 0, "SM2PublicKey", &sm2_key);
+
+	return 0;
+}