Add a <manifest-server> URL so 'repo sync -s' will work.

This points at the autobuilder and allows you to smart-sync to the latest
successfully-built version.

Change-Id: Ie623dcc6e87047509b95ad9e32e389c6762e58ab
diff --git a/default.xml b/default.xml
index d0f0c4f..4e82a07 100644
--- a/default.xml
+++ b/default.xml
@@ -5,6 +5,8 @@
     name="gfiber-internal"
     review="https://gfiber-internal-review.googlesource.com/"/>
   <default remote="gfiber-internal" revision="master" sync-j="12"/>
+  <manifest-server
+    url="http://gfiber-autobot.mtv.corp.google.com:8000/manifest"/>
 
   <project name="toolchains/bruno"/>
   <project name="kernel/bruno"/>