Add platform/build to manifest

Bug:  112467361

default.xml

@@ -8,6 +8,7 @@
            remote="aosp"
            sync-j="4" />
 
+  <project path="build" name="platform/build" />
   <project path="tools/buildSrc" name="platform/tools/buildSrc">
     <copyfile src="base/build.gradle"      dest="tools/build.gradle" />
     <copyfile src="base/settings.gradle"   dest="tools/settings.gradle" />