Fix __must_check compile on older compilers

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index d8911ef..92e6a88 100644
--- a/fio.h
+++ b/fio.h
@@ -13,6 +13,7 @@
 #include <string.h>
 #include <getopt.h>
 
+#include "compiler.h"
 #include "list.h"
 #include "rbtree.h"
 #include "md5.h"