libxml2: explicitly state zlib dir

Pass the zlib directory prefix to --with-zlib otherwise it expands into
absolute (distribution) directories leading to bad test results in
configure. Fixes:
http://autobuild.buildroot.net/results/303/303d6d165c5423944122bf430575c018479733f8/build-end.log



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>