Fix strict error in UploadStashFile

Fix strict error in UploadStashFile

Strict standards: Declaration of UploadStashFile::thumbName() should be compatible with that of File::thumbName() in /var/www/wiki/mediawiki/core/includes/upload/UploadStash.php on line 498

Call Stack:
    0.0004     645672   1. {main}() /var/www/wiki/mediawiki/core/index.php:0
    0.0756    5904264   2. MediaWiki->run() /var/www/wiki/mediawiki/core/index.php:59
    0.0756    5904264   3. MediaWiki->main() /var/www/wiki/mediawiki/core/includes/Wiki.php:446
    0.0797    6328272   4. MediaWiki->performRequest() /var/www/wiki/mediawiki/core/includes/Wiki.php:536
    0.0960    7042800   5. SpecialPageFactory::executePath() /var/www/wiki/mediawiki/core/includes/Wiki.php:290
    0.1041    8003528   6. SpecialPage->run() /var/www/wiki/mediawiki/core/includes/SpecialPageFactory.php:494
    0.1041    8003528   7. SpecialSpecialpages->execute() /var/www/wiki/mediawiki/core/includes/SpecialPage.php:611
    0.2082   17300016   8. SpecialSpecialpages->getPageGroups() /var/www/wiki/mediawiki/core/includes/specials/SpecialSpecialpages.php:42
    0.2083   17300016   9. SpecialPageFactory::getUsablePages() /var/www/wiki/mediawiki/core/includes/specials/SpecialSpecialpages.php:54
    0.4498   30176624  10. SpecialPageFactory::getPage() /var/www/wiki/mediawiki/core/includes/SpecialPageFactory.php:372
    0.4527   30373096  11. SpecialUploadStash->__construct() /var/www/wiki/mediawiki/core/includes/SpecialPageFactory.php:344
    0.4861   33604352  12. FileRepo->getUploadStash() /var/www/wiki/mediawiki/core/includes/specials/SpecialUploadStash.php:52
    0.4861   33604832  13. AutoLoader::autoload() /var/www/wiki/mediawiki/core/includes/AutoLoader.php:0
    0.4898   33952264  14. require('/var/www/wiki/mediawiki/core/includes/upload/UploadStash.php') /var/www/wiki/mediawiki/core/includes/AutoLoader.php:1150

Change-Id: Iee2aae16df804f42cbae9c21c870781a08ad3850