chore: upgrade cpp-httplib from v0.27.0 to v0.28.0 (ggml-org#17513)

Author: o7si

scripts/sync_vendor.py

@@ -16,7 +16,7 @@
     # "https://github.com/mackron/miniaudio/raw/refs/tags/0.11.23/miniaudio.h": "vendor/miniaudio/miniaudio.h",
     "https://github.com/mackron/miniaudio/raw/669ed3e844524fcd883231b13095baee9f6de304/miniaudio.h": "vendor/miniaudio/miniaudio.h",
 
-    "https://raw.githubusercontent.com/yhirose/cpp-httplib/refs/tags/v0.27.0/httplib.h": "vendor/cpp-httplib/httplib.h",
+    "https://raw.githubusercontent.com/yhirose/cpp-httplib/refs/tags/v0.28.0/httplib.h": "vendor/cpp-httplib/httplib.h",
 }
 
 for url, filename in vendor.items():