Specify revision for superproject since default is different from manifest branch

The current superproject for this manifest gets created with the same branch name. Since the default revision in this manifest is for a different branch, we must manually specify the revision on the superproject element.

Change-Id: I854f3f04310d086dc7d2a5079f54540974b416b3
diff --git a/default.xml b/default.xml
index b35a276..ae94cc6 100644
--- a/default.xml
+++ b/default.xml
@@ -3,7 +3,7 @@
   <remote  name="aosp" fetch=".." review="https://android-review.googlesource.com/" />
   <default revision="master-kernel-build-2021" remote="aosp" sync-j="4" />
 
-  <superproject name="kernel/superproject" remote="aosp"/>
+  <superproject name="kernel/superproject" remote="aosp" revision="common-android-4.14-stable" />
 
   <project path="build" name="kernel/build" />
   <project path="common" name="kernel/common" revision="android-4.14-stable" />