print warning is NDEBUG is not defined

examples/llama-bench/llama-bench.cpp

@@ -719,6 +719,10 @@ void llama_null_log_callback(enum llama_log_level level, const char * text, void
 }
 
 int main(int argc, char ** argv) {
+#ifndef NDEBUG
+    fprintf(stderr, "warning: NDEBUG is not defined, performance may be affected\n");
+#endif
+
     cmd_params params = parse_cmd_params(argc, argv);
 
     // initialize llama.cpp