message: Fix external policy build
diff: @@ -764,7 +764,7 @@ class CacheManager : public CacheManagerInterface {
  typedef std::set<std::string> UnpairedDevices;
  UnpairedDevices is_unpaired_;
  - sync_primitives::Lock cache_lock_;
  + mutable sync_primitives::Lock cache_lock_;
  sync_primitives::Lock unpaired_lock_;
  typedef std::map<std::string, Permissions> AppCalculatedPermissions;
  