The host build requires libxml2-dev. This commit adds the dependency for
libxml2/host, as the host system may not have it installed. This also
avoids using the host's xml2-config (in /usr/bin for instance) while
linking to libraries in staging_dir/hostpkg.

ldd staging_dir/hostpkg/bin/xsltproc | grep xml2
	libxml2.so.2 => /home/sk/tmp/openwrt/staging_dir/hostpkg/lib/libxml2.so.2 (0x00007fcc0644c000)

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>