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

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

The code that causes the issue is as the following:
third_party/yasm/tools/genmacro/genmacro.c:43:9: error: variable 'line'
set but not used

Bug: 97838
Change-Id: Ie46f79f8d4a31cadf25e215be6309f78ac214ef6
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/yasm/+/671149
Reviewed-by: Shai Barack <shayba@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 8ec41f2..959d8ac 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -45,6 +45,7 @@
       "-std=gnu99",
       # yasm does not comply with -Wimplicit-fallthrough, so disable it.
       "-Wno-implicit-fallthrough",
+      "-Wno-unused-but-set-variable",
     ]
     configs = [ "//build/config:Wno-conversion" ]
   }