Fix compilation of libxml2

Compilation of libxml2 on some distributions is problematic ( atleast
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>