efl: add missing select of zlib

efl: add missing select of zlib

The efl package has zlib in its dependencies, but does not select it,
which causes a dependency check error with the following defconfig:

BR2_arm=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-arm-full-2016.05-1162-g94c7298.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_4_8=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_10=y
BR2_TOOLCHAIN_EXTERNAL_LOCALE=y
BR2_TOOLCHAIN_EXTERNAL_INET_RPC=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_INIT_NONE=y
BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_EUDEV=y
BR2_SYSTEM_BIN_SH_NONE=y
BR2_PACKAGE_EFL=y

This commit fixes this by selecting the zlib package at the Config.in
level.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>