warning fix

format string is not a string literal (potentially insecure) [-Wformat-security]

test/test_ipc_c_api.cpp

@@ -47,7 +47,7 @@ main(int /* argc */, char** /* argv */)
     auto hline = "\n------------------------\n";
     for (int i = 2; i <= 5; ++i) {
         std::this_thread::sleep_for(500ms);
-        printf(hline);
+        printf("%s", hline);
 
         {
             ipc_write_int(client, &i, 1);
@@ -60,7 +60,7 @@ main(int /* argc */, char** /* argv */)
         }
     }
 
-    printf(hline);
+    printf("%s", hline);
     printf("[c-client] finish the work\n\n");
 
     return 0;