[build] build libxml2 earlier for osx
[build] build libxml2 earlier for osx

BansheeMediaPlayer/bockbuild,bl8/bockbuild,mono/bockbuild,BansheeMediaPlayer/bockbuild,bl8/bockbuild,BansheeMediaPlayer/bockbuild,mono/bockbuild,bl8/bockbuild