[mygames] Fixed: MAME games weren't being shown in MyGames window

XBMC downloads a list of supported extensions from the remote add-ons repo. Zip files are treated as directories and scanned recursively; if they don't contain valid extensions (such as MAME arcade games), the entire zip will be missing from the MyGames window. Skipping the recursive scan always shows zip files (note: entering the zip will show an empty folder) and speeds up directory listing as a nice side effect.