add vold to "shutdown critical"

bug: 37626581
Test: reboot and check last kmsg

(cherry picked from commit 41156c5035efb4aa5212963ac028b7889feda3f8)

Change-Id: I363dbbe58d2cb46b521ebdcea85c507679f27206
diff --git a/vold.rc b/vold.rc
index 53ea7fc..87e2fd8 100644
--- a/vold.rc
+++ b/vold.rc
@@ -6,3 +6,5 @@
     socket cryptd stream 0660 root mount
     ioprio be 2
     writepid /dev/cpuset/foreground/tasks
+    shutdown critical
+