Add another status message to flatpak-builder

Add a message for the 'rewriting the desktop file' step.
This helps in providing context for error messages when something
goes wrong.

builder/builder-manifest.c

@@ -1504,6 +1504,7 @@ builder_manifest_cleanup (BuilderManifest *self,
           gsize desktop_size;
           g_auto(GStrv) desktop_keys = NULL;
 
+          g_print ("Rewriting contents of %s\n", desktop_basename);
           if (!g_file_load_contents (desktop, NULL,
                                      &desktop_contents, &desktop_size, NULL, error))
             {