fixing ajax call to getstoragestats.php

fixes stable5:
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[Mon Dec 02 09:47:31 2013] [error] [client ::1] PHP Fatal error:  Class 'OCP\\JSON' not found in /home/deepdiver/Development/ownCloud/stable5/apps/files/ajax/getstoragestats.php on line 12, referer: http://localhost/oc/stable5/index.php/apps/files
[Mon Dec 02 09:47:31 2013] [error] [client ::1] PHP Stack trace:, referer: http://localhost/oc/stable5/index.php/apps/files
[Mon Dec 02 09:47:31 2013] [error] [client ::1] PHP   1. {main}() /home/deepdiver/Development/ownCloud/stable5/apps/files/ajax/getstoragestats.php:0, referer: http://localhost/oc/stable5/index.php/apps/files