Wrap windows-only C files in #ifdef _WIN32

This should make some scripts and IDEs happier.

src/common/compat_winthreads.c

@@ -3,6 +3,8 @@
  * Copyright (c) 2007-2015, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
+#ifdef _WIN32
+
 #include "compat.h"
 #include <windows.h>
 #include <process.h>
@@ -194,3 +196,5 @@ tor_threads_init(void)
   set_main_thread();
 }
 
+#endif
+