[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>
diff --git a/BUILD.gn b/BUILD.gn
index dfa95f4..9c3a784 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -37,6 +37,7 @@
     "-Wno-implicit-function-declaration",
     "-Wno-conversion",
     "-Wno-implicit-fallthrough",
+    "-Wno-unused-but-set-variable",
   ]
 }