[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>