Decompress zipped XML files ourself instead of letting libxml2 do it.
As of version 2.9.1 it has a bug that causes it to fail to decompress certain files.
See https://bugzilla.gnome.org/show_bug.cgi?id=712528 for more info.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23412 57a11ea4-9604-0410-9ed3-97b8803252fd