util: use _WIN32 instead of WIN32

MinGW defines only _WIN32, but doesn't have fcntl, so we need to use the
windows path.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Acked-by: Kristian H. Kristensen <hoegsberg@google.com>

src/util/anon_file.c

@@ -27,7 +27,7 @@
  * Based on weston shared/os-compatibility.c
  */
 
-#ifndef WIN32
+#ifndef _WIN32
 #include "anon_file.h"
 
 #include <unistd.h>