stoken: Explicitly link with zlib

stoken: Explicitly link with zlib

Fixes build issues with external toolchains:
/home/florian/dev/toolchains/stbgcc-4.8-1.5/bin/../lib/gcc/mipsel-linux-gnu/4.8.5/../../../../mipsel-linux-gnu/bin/ld:
warning: libz.so.1, needed by
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libxml2.so,
not found (try using -rpath or -rpath-link)
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzopen64'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzdirect'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzclose'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzwrite'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzdopen'
/home/florian/dev/openwrt/trunk/staging_dir/target-mipsel-unknown-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzread'
collect2: error: ld returned 1 exit status

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>