add vold to "shutdown critical"

bug: 37626581
Test: reboot and check last kmsg
Change-Id: I2c186c20a4afc342eaae420175013c80219a5ee5
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
+