server : fix bad fmt, size() is a size_type (ggml-org#17735)

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

Author: angt

tools/server/server-common.cpp

@@ -791,7 +791,7 @@ static void handle_media(
         SRV_INF("downloading image from '%s'\n", url.c_str());
         auto res = common_remote_get_content(url, params);
         if (200 <= res.first && res.first < 300) {
-            SRV_INF("downloaded %ld bytes\n", res.second.size());
+            SRV_INF("downloaded %zu bytes\n", res.second.size());
             raw_buffer data;
             data.insert(data.end(), res.second.begin(), res.second.end());
             out_files.push_back(data);