ipsec-tools: fix static linking

As with other packages that use openssl and don't use pkg-config to
pick up the deps it fails to build for static scenarios.
So fix it by adding the zlib link option to LIBS.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>