add libxml2's pkgconfig to avoid having /usr/lib64 in bin/neon-config, lib/libneon.la and lib/pkgconfig/neon.pc.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@40567 20353a03-c40f-0410-a6d1-a30d3c3de9de