libplist: Explicitly link with zlib

libplist: Explicitly link with zlib

Fixes build errors with external toolchains that don't automatically
append STAGING_DIR to the library search path:

/opt/toolchains/stbgcc-4.8-1.5/lib/gcc/mipsel-linux-gnu/4.8.5/../../../../mipsel-linux-gnu/bin/ld:
warning: libz.so.1, needed by
/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/usr/lib/libxml2.so,
not found (try using -rpath or -rpath-link)
/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzopen64'
/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzdirect'
/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzclose'
/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzwrite'
/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzdopen'
/local/users/fainelli/openwrt/trunk/staging_dir/target-mipsel-linux-gnu_glibc/usr/lib/libxml2.so:
undefined reference to `gzread'
collect2: error: ld returned 1 exit status
make[5]: *** [plistutil] Error 1
make[5]: Leaving directory
`/local/users/fainelli/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/libplist-1.13/tools'

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