"NOMINMAX" is not used [-Werror=unused-macros]

So let's let this for Windows only

Change-Id: I1f76e03b0e3431a6954a75f5cb247ef1a1644a28
diff --git a/sal/rtl/bootstrap.cxx b/sal/rtl/bootstrap.cxx
index 0d86131..0ddeb5f 100644
--- a/sal/rtl/bootstrap.cxx
+++ b/sal/rtl/bootstrap.cxx
@@ -16,8 +16,9 @@
 *   except in compliance with the License. You may obtain a copy of
 *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
 */

#ifdef WINNT
#define NOMINMAX
#endif

#include <config_features.h>
#include <config_folders.h>