Provide default value for id to fix warning

src/libstore/path-info.hh

@@ -42,7 +42,7 @@ struct UnkeyedValidPathInfo
     StorePathSet references;
     time_t registrationTime = 0;
     uint64_t narSize = 0; // 0 = unknown
-    uint64_t id; // internal use only
+    uint64_t id = 0; // internal use only
 
     /**
      * Whether the path is ultimately trusted, that is, it's a