Set copyright holders displayed in notices separately from the package name

This helps avoid accidental removal of upstream copyright names

src/util.h

@@ -242,4 +242,6 @@ template <typename Callable> void TraceThread(const char* name,  Callable func)
     }
 }
 
+std::string CopyrightHolders();
+
 #endif // BITCOIN_UTIL_H