glib-host depends on libxml2-host for xmllint

Fixes Issue #2171

Building on Ubuntu 13.04 x86_64 was failing with a version conflict for liblzma.

/usr/bin/xmllint: /root/openelec/dev/OpenELEC.tv/build.OpenELEC-ION.x86_64-devel/toolchain/lib/liblzma.so.5: version `XZ_5.0' not found (required by /usr/lib/x86_64-linux-gnu/libxml2.so.2)

glib-host needs xmllint to parse some test xml file.  xmllint was being found in my /usr/bin path, which required liblzma which was found in $BUILD/toolchain/lib/liblzma.so

My local /usr/lib/liblzma is 5.0.0, but the OE liblzma is 5.0.5 so xmllint would fail to start. 

This patch just adds libxml2-host as a dependency to glib-host.