php-memcached: make zlib directory explicit

Otherwise it fails to find in some cases. Fixes:
http://autobuild.buildroot.net/results/993/99342e74d0866a7acddbf9da66aafbfa4f3c12f1/



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