Add external/r8 to repo manifest file.

BUG: 62716066
Change-Id: I03c41760c8738b5ca5ddbcabf0f0343ae7fd6a3d

default.xml

@@ -276,6 +276,7 @@
   <project path="external/protobuf" name="platform/external/protobuf" groups="pdk" />
   <project path="external/python/cpython2" name="platform/external/python/cpython2" groups="pdk" />
   <project path="external/python/cpython3" name="platform/external/python/cpython3" groups="pdk" />
+  <project path="external/r8" name="platform/external/r8" groups="pdk" />
   <project path="external/regex-re2" name="platform/external/regex-re2" groups="pdk" />
   <project path="external/replicaisland" name="platform/external/replicaisland" groups="pdk" />
   <project path="external/rmi4utils" name="platform/external/rmi4utils" groups="pdk" />