campaignd: Include core= attribute from _server.pbl

Closes #5664.

changelog.md

@@ -1,6 +1,7 @@
 ## Version 1.16.0+dev
  ### Add-ons client
  ### Add-ons server
+   * Fixed `core=` attribute in _server.pbl being completely ignored (issue #5664).
  ### Campaigns
    * Tutorial
      * Use a portrait on the Campaigns menu.

src/server/campaignd/server.cpp

@@ -1470,7 +1470,7 @@ void server::handle_upload(const server::request& req)
 
 	addon.copy_attributes(upload,
 		"title", "name", "author", "description", "version", "icon",
-		"translate", "dependencies", "type", "tags", "email");
+		"translate", "dependencies", "core", "type", "tags", "email");
 
 	const std::string& pathstem = "data/" + name;
 	addon["filename"] = pathstem;