build.xml: bundle a 64x64 PNG icon in the generated zip file

git-svn-id: http://micropolis.googlecode.com/svn/trunk@796 d9718cc8-9f43-0410-858b-315f434eb58c
TODO- document an issue with draw-bridges

git-svn-id: http://micropolis.googlecode.com/svn/trunk@797 d9718cc8-9f43-0410-858b-315f434eb58c