zlib-ng: add bash as a build-time dependency for make

Upon reading the APKBUILD file for zlib-ng, and having tried to manually configure the package on a host system where bash does not exist, I noticed that their configure script hardcodes bash in the shebang #!/usr/bin/env bash.

Apparently they are not using any bashisms, so simply changing the shebang to #!/bin/sh works, but I think it would be wise to include bash as a make depend (similar to how libcap depends on bash for mkcapshdoc.sh, and toybox for scripts/genconfig.sh).