[build] Suppress -Wunused-but-set-variable in zlib

This patch suppresses -Wunused-but-set-variable warning in
zlib third party code that does not comply with.

The code that causes the issue is as the following:
third_party/zlib/contrib/minizip/zip.c:1476:21: error: variable 'bBreak'
set but not used

Bug: 96410
Change-Id: Ie42a969578497e60d013ac098e4fc4914bb499eb
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/zlib/+/663833
Reviewed-by: Petr Hosek <phosek@google.com>