libxml2: fix compilation on archlinux and gentoo

Compilation of libxml2 on some distributions is problematic (at least
archlinux) for OpenWrt. This commit fixes the issue. Issue is caused
because configuration for some reason does not find gzopen from zlib.
This patch issues linker to include zlib anyway, if host system doesn't
have this issue, it is not a problem as linker should not link libs
twice anyway.

Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>

[ Slightly modified commit message ]
Signed-off-by: Michael Heimpold <mhei@heimpold.de>