Add sync-j default to manifest.

Similar to BR2_JLEVEL, this changes the default option for parallel
jobs executed by repo sync, speeding up the previous linear sync.
diff --git a/default.xml b/default.xml
index f99d540..88be6ad 100644
--- a/default.xml
+++ b/default.xml
@@ -4,7 +4,7 @@
     fetch="ssh://wavejumper.i.corp.google.com:29418"
     name="wavejumper"
     review="http://wavejumper.i.corp.google.com/"/>
-  <default remote="wavejumper" revision="master"/>
+  <default remote="wavejumper" revision="master" sync-j="6"/>
 
   <project name="manifest"                      path="manifest"/>
   <project name="toolchains-uno"                path="toolchains/uno"/>