use autoloader in ext_localconf.php

tx_languagevisibility_hooks_t3lib_page and
tx_languagevisibility_hooks_tslib_menu are instanciate quite often.
Having TYPO3 parse the file string in front of the class name multiple
times is needless overhead.

Change-Id: Ia9235c6f2956999679cd5cc440dccdc8be6a884a
Reviewed-on: https://review.typo3.org/30097
Reviewed-by: Christian Zenker
Tested-by: Christian Zenker