Make td_verror() use an unsigned type

Get rid of warnings stating the input can't be negative.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio.h b/fio.h
index 3df5bd9..1445913 100644
--- a/fio.h
+++ b/fio.h
@@ -351,7 +351,7 @@
 
 #define __td_verror(td, err, msg, func)					\
 	do {								\
-		int ____e = (err);						\
+		unsigned int ____e = (err);				\
 		if ((td)->error)					\
 			break;						\
 		(td)->error = ____e;					\