Don't build off-by-default loplugin:unusedfields on Windows for now

...due to missing sys/file.h

Change-Id: I7cfd64c5355d9fdbb85320f876c277a408be9352
Reviewed-on: https://gerrit.libreoffice.org/47675
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/unusedfields.cxx

@@ -7,6 +7,8 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
+#if !defined _WIN32 //TODO, #include <sys/file.h>
+
 #include <cassert>
 #include <string>
 #include <iostream>
@@ -989,4 +991,6 @@ loplugin::Plugin::Registration< UnusedFields > X("unusedfields", false);
 
 }
 
+#endif
+
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */