sqlcipher: fix static linking

It uses openssl which for buildroot mandates libz, but doesn't link
against it so it fails. Fixes:
http://autobuild.buildroot.net/results/514/5145617f7c3cece933c845da3c9836d80d062bb6/



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