Fix warnings in bootstrap lib on Windows

This amends commit 174af05400.

Pick-to: 6.3
Change-Id: Ifc11b3c332f2b7fd16074420a0d730997ae13395
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Ievgenii Meshcheriakov <ievgenii.meshcheriakov@qt.io>

src/corelib/io/qfilesystemengine_win.cpp

@@ -593,6 +593,9 @@ QNativeFilePermissions::QNativeFilePermissions(std::optional<QFileDevice::Permis
     sa.bInheritHandle = FALSE;
 
     isNull = false;
+#else
+    Q_UNUSED(perms);
+    Q_UNUSED(isDir);
 #endif // QT_CONFIG(fslibs)
     ok = true;
 }