Update IO engine version

Commit 225ba9e3 changed the layout of struct io_u, so
update the engine version to indicate that external
engines need to be recompiled.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/ioengine.h b/ioengine.h
index 7e0707b..6e3c717 100644
--- a/ioengine.h
+++ b/ioengine.h
@@ -15,7 +15,7 @@
 #include <guasi.h>
 #endif
 
-#define FIO_IOOPS_VERSION	17
+#define FIO_IOOPS_VERSION	18
 
 enum {
 	IO_U_F_FREE		= 1 << 0,