Don't update zlib files when syncing libgit2

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Use zconf.h from Rtools goodies

* src/zlib/zconf.h: Use zconf.h from
  http://www.stats.ox.ac.uk/pub/Rtools/goodies/multilib/local320.zip

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