Fix bogus 'may be used uninitialized' warning from older gcc versions

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 8863fa7..f2406e8 100644
--- a/io_u.c
+++ b/io_u.c
@@ -882,7 +882,11 @@
 static void io_completed(struct thread_data *td, struct io_u *io_u,
 			 struct io_completion_data *icd)
 {
-	unsigned long usec;
+	/*
+	 * Older gcc's are too dumb to realize that usec is always used
+	 * initialized, silence that warning.
+	 */
+	unsigned long uninitialized_var(usec);
 
 	dprint_io_u(io_u, "io complete");