fileutils: fix compilation on windows

Change-Id: I652ffd4a1d3a6ed0adf6402cdf55cc97beb811a3
diff --git a/include/fileutils.h b/include/fileutils.h
index 8362cac..80efe42 100644
--- a/include/fileutils.h
+++ b/include/fileutils.h
@@ -24,6 +24,10 @@
 #include <ios>
 #include <filesystem>
 
+#ifndef _WIN32
+#include <sys/stat.h>               // mode_t
+#endif
+
 namespace dhtnet {
 namespace fileutils {