Add libxml2 for the host

Some other packages, such as shared-mime-info, might need libxml2 and
related tools (xmllint, etc.) to be available on the host. This patch
modifies tthe libxml2 Makefile to compile libxml2 for the host, in
$(HOST_DIR).

Patch from Sven Neumann <s.neumann@raumfeld.com>

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>