Twenty Fifteen: Add a transparent border to a link in the footer to avoid jumpiness on hover.

props iamtakashi.
fixes #30332.
Built from https://develop.svn.wordpress.org/trunk@30331


git-svn-id: http://core.svn.wordpress.org/trunk@30330 1a063a9b-81f0-0310-95a4-ce76da25c4cd