When listing files from a directory, only list those that contain a version file.

Any empty or non-style directories would be included in the list, but would
fail to open, leading to the real style being opened.  Since that style would
be already in the list, you get a message about a duplicate.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3137 25d90789-57f7-4ee0-8453-03a3dfeeeb22