Bump IO engine version number

Commit 1f440ece6127 modified the ->getevents() hook to have
a const timespec, so we should increment the engine version
number to catch that in external engines.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/ioengine.h b/ioengine.h
index dfe84ac..85923fc 100644
--- a/ioengine.h
+++ b/ioengine.h
@@ -15,7 +15,7 @@
 #include <guasi.h>
 #endif
 
-#define FIO_IOOPS_VERSION	20
+#define FIO_IOOPS_VERSION	21
 
 enum {
 	IO_U_F_FREE		= 1 << 0,