missing extension-list-labs

mergeMessageFileList.php can be passed files using the
$wgExtensionEntryPointListFiles global variable.  Whenever a file is
missing it bails out with an error.

The extension-list-labs was removed previously by 976f2e9, causing beta
to no more update the messages :-(

Change-Id: Ib97f5b882f8654b926bd277dae1266543ec26f6d