Use #!/usr/bin/env to allow for interpreters in different directories.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/tools/genfio b/tools/genfio
index 11cf9a4..4d32d13 100755
--- a/tools/genfio
+++ b/tools/genfio
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 #  Copyright (C) 2013 eNovance SAS <licensing@enovance.com>
 #  Author: Erwan Velu  <erwan@enovance.com>