Update zlib source code

* Use zlib source code version 1.2.8 from www.zlib.net. (md5sum
  zlib-1.2.8.tar.gz 44d667c142d7cda120332623eab69f40 zlib-1.2.8.tar.gz)

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>