FIO Windows update

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/fio.h b/fio.h
index c04ad8b..9598858 100644
--- a/fio.h
+++ b/fio.h
@@ -14,6 +14,8 @@
 #include <inttypes.h>
 #include <assert.h>
 
+struct thread_data;
+
 #include "compiler/compiler.h"
 #include "flist.h"
 #include "fifo.h"