[build] Suppress -Wunused-but-set-variable in openssh-portable

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

The code that causes the issue is as the following:
third_party/openssh-portable/kex.c:1172:14: error: variable 'i' set but
not used

Bug: 96410
Change-Id: I5c49f33a9105308244370b5ca424edce84c7f162
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/openssh-portable/+/663831
Fuchsia-Auto-Submit: Gulfem Savrun Yeniceri <gulfem@google.com>
Reviewed-by: Petr Hosek <phosek@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
diff --git a/BUILD.gn b/BUILD.gn
index d3cb5b6..b6687ff 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -44,6 +44,7 @@
     "-Wno-sign-compare",
     "-Wno-strict-prototypes",
     "-Wno-write-strings",
+    "-Wno-unused-but-set-variable",
   ]
 
   configs = [