mysql: depend on zlib to not build own one

When mysql does not detect zlib during configure then their copy of
zlib 1.2.3 is built. That leads to problems during rootfs creation:

|  * check_data_file_clashes: Package libz-staticdev wants to install file /home/hrw/HDD/devel/canonical/aarch64/openembedded/build/tmp-eglibc/work/genericarmv8-oe-linux/linaro-image-lamp/1.0-r0/rootfs/usr/lib/libz.a
|       But that file is already provided by package  * libmysqlclient-staticdev
|  * opkg_install_cmd: Cannot install package libz-staticdev.

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>