Add external/error_prone to the manifest

Test: none, contains no build files
Bug: 64489631
Change-Id: I56d9cbe9fe194c3b68872db46a4cfb3ac5f0a787

default.xml

@@ -133,6 +133,7 @@
   <project path="external/eigen" name="platform/external/eigen" groups="pdk" />
   <project path="external/elfutils" name="platform/external/elfutils" groups="pdk" />
   <project path="external/emma" name="platform/external/emma" groups="pdk-cw-fs,pdk-fs" />
+  <project path="external/error_prone" name="platform/external/error_prone" groups="pdk" />
   <project path="external/esd" name="platform/external/esd" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/expat" name="platform/external/expat" groups="pdk" />
   <project path="external/eyes-free" name="platform/external/eyes-free" groups="pdk-cw-fs,pdk-fs" />