bind: fix build failure for static+openssl

Bind just links to openssl libraries without checking it's dependencies.
This works when doing shared builds since the linker picks the deps up,
but fails badly on static builds.
So just define LIBS="-lz" when openssl is enabled. Fixes:
http://autobuild.buildroot.net/results/a9a166f932e0b6727ae8e470ce748418797875b9/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>