Rename libmetacity-private3 package to libmetacity-private3a.

There was an ABI break upstream, which is legitimate from upstream
point of view (as 3.17.x was an unstable branch), but we shipped
3.17.x in unstable so we need to adopt accordingly.

git-svn-id: svn://anonscm.debian.org/pkg-gnome/desktop/unstable/metacity@46094 db0db5de-e4c8-0310-9441-90abf70311f7