server-models.cpp: add missing <filesystem> (#18000)

Fixes: https://github.com/ggml-org/llama.cpp/issues/17999

tools/server/server-models.cpp

@@ -16,6 +16,7 @@
 #include <atomic>
 #include <chrono>
 #include <queue>
+#include <filesystem>
 
 #ifdef _WIN32
 #include <winsock2.h>